diff --git a/backend/app/bodylimit.py b/backend/app/bodylimit.py
index 0008ee62..9bf8ce33 100644
--- a/backend/app/bodylimit.py
+++ b/backend/app/bodylimit.py
@@ -53,31 +53,32 @@ DEFAULT_MAX_BODY_BYTES = 1024 * 1024
 #
 # These are RAISED CEILINGS, NOT EXEMPTIONS, and the difference is the whole point. An exemption
 # would mean "this route is counted by something else" — which is only true when the route's own
-# limit applies to the bytes as they ARRIVE. That holds for the screenshot route (no declared body
-# parameter: `capped_body` streams it, and the owner check resolves first) and it does NOT hold for
-# the upload route: `file: UploadFile = File(...)` is a declared body, so FastAPI parses and spools
-# the whole multipart part to a temp file BEFORE the first dependency runs — `require_human`
-# included. Its in-handler cap therefore measures bytes that have already landed. Left uncounted,
-# an anonymous caller could stream without bound into the container's tmpfs; counted here, the
-# process refuses at the same number the route itself enforces, before any of it is stored.
+# limit applies to the bytes as they ARRIVE. That, in turn, is only true of a route with NO DECLARED
+# BODY PARAMETER: FastAPI parses and spools a declared body BEFORE the first dependency runs, so a
+# route that declares one has already stored the bytes by the time its own cap — or its auth — gets
+# a say.
+#
+# Both listed routes now satisfy that. The screenshot route always did (`capped_body` streams it,
+# and the owner check resolves first). The upload route did NOT until P1: it declared
+# `file: UploadFile = File(...)`, so an anonymous caller could make the process write the whole
+# per-file allowance to temp before `require_human` refused them — this middleware bounded that
+# number but could not stop the writing. It now reads its own body from a dependency declared after
+# the auth one, so its in-handler cap measures bytes as they arrive, like everything else here.
+#
+# That rule is no longer only written here: `test_body_limit.py` parses the routes and fails the
+# gate if any route in this dict declares a body parameter. A comment could not have stopped the
+# defect it describes; the test can.
 #
 # A route belongs here ONLY IF ITS OWN CAP IS LARGER THAN THE DEFAULT. That is why
 # `POST /api/diagnostics/events` is absent: its `capped_body` limit is 512 KiB, already below the
 # default, so listing it could only widen what it accepts.
-# What a multipart envelope costs on top of the file itself: the boundary lines, the part headers
-# and the sibling `display_name` field. The edge counts RAW BODY bytes while the upload route counts
-# the DECODED file, so without this allowance a file of exactly the configured maximum — the number
-# the admin page advertises, and the first thing anyone testing the limit reaches for — is refused
-# one layer before the route that would have accepted it. Generous on purpose: its job is to keep
-# the two layers from disagreeing at the boundary, not to be a second limit.
-MULTIPART_ENVELOPE_ALLOWANCE = 64 * 1024
-
 ROUTE_MAX_BODY_BYTES: dict[tuple[str, str], int] = {
-    # Ingesting a local video file. `sysconfig`'s `download_upload_max_bytes` is bounded so it can
-    # never exceed this (see sysconfig.SPECS), which is what stops an admin from raising the route's
-    # own cap past the edge and collecting 413s from a layer the admin page never mentions.
-    ("POST", "/api/downloads/upload"): settings.download_upload_max_bytes
-    + MULTIPART_ENVELOPE_ALLOWANCE,
+    # Ingesting a local file. The body IS the file since P1 — no multipart envelope to allow for —
+    # so this is exactly the number the route enforces. `sysconfig`'s `download_upload_max_bytes` is
+    # bounded so it can never exceed it (see sysconfig.SPECS), which is what stops an admin from
+    # raising the route's own cap past the edge and collecting 413s from a layer the admin page
+    # never mentions.
+    ("POST", "/api/downloads/upload"): settings.download_upload_max_bytes,
     ("POST", "/api/debug-sessions/{sid}/screenshots"): debugsession.SCREENSHOT_MAX_BYTES,
 }
 
diff --git a/backend/app/routes/_common.py b/backend/app/routes/_common.py
index b7e99a55..19205df3 100644
--- a/backend/app/routes/_common.py
+++ b/backend/app/routes/_common.py
@@ -6,7 +6,7 @@ and "fetch a row this user owns or 404" idioms have one definition, one status c
 one existence-leak policy.
 """
 
-from typing import Any, TypeVar
+from typing import IO, Any, TypeVar
 
 from fastapi import HTTPException, Request
 from sqlalchemy.orm import Session
@@ -79,6 +79,30 @@ async def capped_body(request: Request, limit: int) -> bytes:
     return bytes(body)
 
 
+async def capped_stream_to(
+    request: Request, out: IO[bytes], limit: int, refusal: HTTPException
+) -> int:
+    """`capped_body`'s sibling for a body too large to hold: the same counting rule, written
+    straight to `out` instead of accumulated, returning the size.
+
+    Two deliberate differences from `capped_body`. It never builds a `bytearray` — the upload route
+    admits 500 MiB by default, so holding the body would trade a disk problem for a memory one. And
+    the refusal is the CALLER'S, passed in rather than assumed: `capped_body`'s flat 413 is right
+    for a screenshot, while the upload route answers 422 `upload_too_large` carrying the admin
+    knob's own number, which is what its dialog shows the user. One counting rule, two answers,
+    neither of them a second copy of the loop.
+
+    The caller owns `out` and owns cleaning it up: a body refused half-way has already written what
+    arrived before the limit was passed."""
+    size = 0
+    async for chunk in request.stream():
+        size += len(chunk)
+        if size > limit:
+            raise refusal
+        out.write(chunk)
+    return size
+
+
 def issue_refused(exc: gitea.GiteaError) -> HTTPException:
     """The refusal an issue-tracker failure (E16) answers as — the send, the preview and the
     admin's connection test alike. Literal reasons, one per case: the frontend's contract test reads
diff --git a/backend/app/routes/downloads.py b/backend/app/routes/downloads.py
index 90e36197..c71380b4 100644
--- a/backend/app/routes/downloads.py
+++ b/backend/app/routes/downloads.py
@@ -9,11 +9,13 @@ serves finished files (range-aware, with the user's custom display name), and sh
 import os
 import re
 import tempfile
+from collections.abc import AsyncIterator
+from dataclasses import dataclass
 from datetime import datetime, timedelta, timezone
 from pathlib import Path
-from urllib.parse import parse_qs, urlparse
+from urllib.parse import parse_qs, unquote, urlparse
 
-from fastapi import APIRouter, Depends, File, Form, HTTPException, Request, UploadFile
+from fastapi import APIRouter, Depends, HTTPException, Request
 from fastapi.responses import FileResponse
 from pydantic import BaseModel
 from sqlalchemy import func, select
@@ -38,7 +40,7 @@ from app.models import (
     MediaAsset,
     User,
 )
-from app.routes._common import get_or_404, not_found, owned_or_404
+from app.routes._common import capped_stream_to, get_or_404, not_found, owned_or_404
 from app.security import hash_password
 from app.userscope import is_messageable_user, messageable_clauses
 from app.utils import iso
@@ -406,61 +408,113 @@ def enqueue_download(
     return _serialize_job(db, job)
 
 
-# Read the upload in 1 MiB chunks so a large file never sits fully in memory, and the size cap is
-# enforced on the ACTUAL bytes read (a client-set Content-Length can't be trusted to gate it).
-_UPLOAD_CHUNK = 1024 * 1024
+# The upload's filename and display name travel as HEADERS, not as multipart parts. That is not a
+# style choice: a DECLARED body parameter (`file: UploadFile = File(...)`) is read and spooled by
+# FastAPI *before the first dependency runs*, so `require_human` could not gate it and an anonymous
+# caller could make the process write the whole per-file allowance to temp, per request (P1; SEC-20
+# bounded the number but could not stop the writing). Reading the body ourselves, from a dependency
+# declared AFTER the auth one, is the only shape that closes it — the same one
+# `POST /api/debug-sessions/{sid}/screenshots` uses, and `test_body_limit.py` now enforces it for
+# every route with a raised ceiling rather than leaving it to a comment like this one.
+#
+# Headers are latin-1 on the wire, so both are percent-encoded by the client and unquoted here; a
+# Hungarian filename would otherwise be unsendable.
+UPLOAD_FILENAME_HEADER = "x-upload-filename"
+UPLOAD_DISPLAY_NAME_HEADER = "x-upload-display-name"
 
 
-@router.post("/upload")
-def upload_download(
-    file: UploadFile = File(...),
-    display_name: str | None = Form(None),
+@dataclass(frozen=True)
+class SpooledUpload:
+    """What the body dependency hands the handler: the bytes already on disk, plus the two names
+    that came alongside them."""
+
+    path: Path
+    size: int
+    user_id: int
+    filename: str
+    display_name: str | None
+
+
+def _header_text(request: Request, name: str) -> str:
+    """One percent-encoded header, decoded. A header that is not valid UTF-8 once unquoted is the
+    caller's doing, not a server fault — it degrades to empty rather than 500ing, and the route's
+    own fallback names the file."""
+    raw = request.headers.get(name, "")
+    try:
+        return unquote(raw, errors="strict").strip()
+    except (UnicodeDecodeError, ValueError):
+        # silent-ok: a mis-encoded header is the caller's own malformed request, and this one is
+        # cosmetic — it names the file. Refusing the upload over it would fail a 500 MiB transfer on
+        # a label; logging it would hand any client a free log line per request.
+        return ""
+
+
+async def spool_upload(
+    request: Request,
     user: User = Depends(require_human),
     db: Session = Depends(get_db),
-) -> dict:
-    """Ingest a local file into the caller's library (E7 S5). Any file type is accepted; media
-    (video/audio) gets the full function set, a generic file is download-only. The file is streamed
-    to a temp path under the download volume (enforcing the per-file cap as it goes), then handed to
-    the non-yt-dlp ingest which probes + places it and returns an already-`done` job.
+) -> AsyncIterator[SpooledUpload]:
+    """Stream the request body to a staging temp file, enforcing the per-file cap as it arrives.
 
-    A sync (threadpool) handler on purpose: the ingest does blocking work — an ffprobe of up to 30s
-    plus sync DB writes — so it must NOT run on the event loop. `file.file` is the sync file object
-    Starlette spools the multipart body into."""
+    `user` is taken but unused: it is the whole point. Dependencies resolve in declaration order, so
+    naming `require_human` here makes the auth check a precondition of reading the body at all — a
+    stranger is refused before a byte moves. The `yield` shape owns the temp file's life: ingest
+    MOVES it into place on success (the unlink is then a no-op), and on any other exit — cap hit,
+    empty, quota, probe or move error, a client that vanished mid-upload — it is removed."""
     cap = sysconfig.get_int(db, "download_upload_max_bytes")
     staging = Path(settings.download_root) / ".staging" / "uploads"
     staging.mkdir(parents=True, exist_ok=True)
     fd, tmp = tempfile.mkstemp(dir=str(staging))
     tmp_path = Path(tmp)
     try:
-        size = 0
         with os.fdopen(fd, "wb") as out:
-            while chunk := file.file.read(_UPLOAD_CHUNK):
-                size += len(chunk)
-                if cap and size > cap:
-                    raise HTTPException(
-                        status_code=422, detail={"code": "upload_too_large", "limit": cap}
-                    )
-                out.write(chunk)
-        if size == 0:
-            raise HTTPException(status_code=400, detail={"code": "upload_empty"})
-        try:
-            job = uploadmod.ingest_upload(
-                db,
-                user.id,
-                filename=file.filename or "upload",
-                src_path=tmp_path,
-                size=size,
-                display_name=(display_name or "").strip() or None,
+            size = await capped_stream_to(
+                request,
+                out,
+                cap,
+                HTTPException(status_code=422, detail={"code": "upload_too_large", "limit": cap}),
             )
-        except quota.QuotaExceeded as e:
-            raise HTTPException(status_code=422, detail=_quota_detail(e))
-        return _serialize_job(db, job)
+        yield SpooledUpload(
+            path=tmp_path,
+            size=size,
+            user_id=user.id,
+            filename=_header_text(request, UPLOAD_FILENAME_HEADER) or "upload",
+            display_name=_header_text(request, UPLOAD_DISPLAY_NAME_HEADER) or None,
+        )
     finally:
-        # Ingest MOVES the temp file into place on success (unlink is then a no-op); on any early exit
-        # — cap hit, empty, quota, probe/move error — this removes the abandoned staging file.
         tmp_path.unlink(missing_ok=True)
 
 
+@router.post("/upload")
+def upload_download(
+    upload: SpooledUpload = Depends(spool_upload),
+    db: Session = Depends(get_db),
+) -> dict:
+    """Ingest a local file into the caller's library (E7 S5). Any file type is accepted; media
+    (video/audio) gets the full function set, a generic file is download-only. The body IS the file
+    (see the header note above); `spool_upload` has already authenticated the caller and streamed it
+    to a temp path under the download volume, so what is left here is the non-yt-dlp ingest which
+    probes + places it and returns an already-`done` job.
+
+    A sync (threadpool) handler on purpose: the ingest does blocking work — an ffprobe of up to 30s
+    plus sync DB writes — so it must NOT run on the event loop. The streaming half is async because
+    reading the body is; keeping them in separate callables is what lets each run where it belongs."""
+    if upload.size == 0:
+        raise HTTPException(status_code=400, detail={"code": "upload_empty"})
+    try:
+        job = uploadmod.ingest_upload(
+            db,
+            upload.user_id,
+            filename=upload.filename,
+            src_path=upload.path,
+            size=upload.size,
+            display_name=upload.display_name,
+        )
+    except quota.QuotaExceeded as e:
+        raise HTTPException(status_code=422, detail=_quota_detail(e))
+    return _serialize_job(db, job)
+
+
 class EnqueueEditIn(BaseModel):
     source_job_id: int = 0
     edit_spec: dict
diff --git a/backend/tests/conftest.py b/backend/tests/conftest.py
index 1c817f11..c25be444 100644
--- a/backend/tests/conftest.py
+++ b/backend/tests/conftest.py
@@ -15,6 +15,7 @@ tests that COMMIT (the whole point of this lane — transaction-ownership behavi
 
 import os
 import shutil
+from urllib.parse import quote
 
 import pytest
 from alembic import command
@@ -248,6 +249,27 @@ def as_user(db, override_dep, _configured):
     return _as
 
 
+@pytest.fixture
+def upload_headers():
+    """Build the headers `POST /api/downloads/upload` takes since P1.
+
+    Shared, because two test modules drive that route and a second copy of this is exactly the drift
+    that lets the wire shape change in one place only. The body IS the file now; the names ride as
+    percent-encoded headers (`quote` mirrors the client's `encodeURIComponent`, since headers are
+    latin-1 and a Hungarian filename would not survive raw)."""
+
+    def _headers(filename: str, display_name: str | None = None) -> dict[str, str]:
+        headers = {
+            "content-type": "application/octet-stream",
+            "x-upload-filename": quote(filename),
+        }
+        if display_name is not None:
+            headers["x-upload-display-name"] = quote(display_name)
+        return headers
+
+    return _headers
+
+
 @pytest.fixture(autouse=True)
 def _reset_rate_limiters():
     """Every RateLimiter is a module-level singleton keyed by client IP, and TestClient always
diff --git a/backend/tests/test_body_limit.py b/backend/tests/test_body_limit.py
index 05bebba7..d19bf2ec 100644
--- a/backend/tests/test_body_limit.py
+++ b/backend/tests/test_body_limit.py
@@ -251,14 +251,57 @@ def test_the_upload_ceiling_and_the_admin_knob_cannot_drift_apart():
     constant. If the knob could be set above the edge, an admin raising it would silently start
     getting 413s from a layer the admin page never mentions.
 
-    The relation is `<=`, not `==`, and the gap is the point: the edge counts RAW body bytes while
-    the route counts the DECODED file, so the edge has to allow for the multipart envelope or a file
-    of exactly the advertised maximum is refused one layer before the route that would take it.
+    Since P1 the two count the SAME bytes — the body is the file, with no multipart envelope
+    wrapped round it — so the old allowance is gone and the edge is the knob's own maximum,
+    exactly. A gap reappearing means someone re-wrapped the upload in an envelope without saying so.
     """
     spec = next(s for s in sysconfig.SPECS if s.key == "download_upload_max_bytes")
     edge = bodylimit.ROUTE_MAX_BODY_BYTES[("POST", "/api/downloads/upload")]
-    assert spec.max is not None and spec.max <= edge
-    assert edge - spec.max == bodylimit.MULTIPART_ENVELOPE_ALLOWANCE
+    assert spec.max == edge
+
+
+def _route_objects(root):
+    """`_declared_routes`' sibling, keeping the route OBJECT so its FastAPI dependant can be read."""
+    found: dict[tuple[str, str], object] = {}
+    stack = [root]
+    while stack:
+        node = stack.pop()
+        for route in getattr(node, "routes", ()) or ():
+            inner = getattr(route, "original_router", None)
+            if inner is not None:
+                stack.append(inner)
+                continue
+            for method in getattr(route, "methods", None) or ():
+                found[(method, route.path)] = route
+    return found
+
+
+@pytest.mark.parametrize(("method", "template"), sorted(bodylimit.ROUTE_MAX_BODY_BYTES))
+def test_no_route_with_a_raised_ceiling_declares_a_body_parameter(method, template):
+    """P1's invariant, and the reason this is a test rather than a comment.
+
+    A raised ceiling is only honest when the route's own cap applies to the bytes AS THEY ARRIVE.
+    FastAPI reads and spools a DECLARED body (`= File(...)`, `= Form(...)`, a Pydantic model)
+    BEFORE the first dependency resolves — so on such a route this middleware's ceiling is the only
+    thing between an anonymous caller and the whole allowance, `require_human` never gets a say, and
+    the route's in-handler cap measures bytes that have already landed. That is exactly what P1
+    closed on the upload route, which sat in `ROUTE_MAX_BODY_BYTES` with a 500 MiB ceiling and a
+    `file: UploadFile = File(...)` body for one release.
+
+    Asked of FastAPI itself (`route.dependant.body_params`) rather than parsed or grepped: the
+    router already knows which parameters it will read the body for, and a shape a regex or an AST
+    walk guesses wrong is precisely how the next route slips through."""
+    from app.main import app
+
+    route = _route_objects(app).get((method, template))
+    assert route is not None, f"{method} {template} is not a route any more"
+    body_params = [p.name for p in getattr(route.dependant, "body_params", ())]
+    assert not body_params, (
+        f"{method} {template} declares a body parameter ({', '.join(body_params)}), so FastAPI "
+        "spools it before the first dependency runs and the raised ceiling is all that stands "
+        "between an anonymous caller and the whole allowance. Read the body yourself, from a "
+        "dependency declared AFTER the auth one — see routes/downloads.py:spool_upload."
+    )
 
 
 def test_a_stored_override_outside_the_bounds_falls_back_on_READ(db):
diff --git a/backend/tests/test_public_unlock_limit.py b/backend/tests/test_public_unlock_limit.py
index ffbdebd0..816575a2 100644
--- a/backend/tests/test_public_unlock_limit.py
+++ b/backend/tests/test_public_unlock_limit.py
@@ -18,12 +18,12 @@ _PW = "open sesame"
 
 
 @pytest.fixture
-def unlock_url(as_user, make_user, tmp_path, monkeypatch):
+def unlock_url(as_user, make_user, tmp_path, monkeypatch, upload_headers):
     monkeypatch.setattr(settings, "download_root", str(tmp_path))
     monkeypatch.setattr(probemod, "probe_media", lambda _p: ("h264", "aac", 320, 240, 5))
     owner = as_user(make_user())
     job = owner.post(
-        "/api/downloads/upload", files={"file": ("clip.mp4", b"fake-video", "video/mp4")}
+        "/api/downloads/upload", content=b"fake-video", headers=upload_headers("clip.mp4")
     )
     assert job.status_code == 200, job.text
     link = owner.post(f"/api/downloads/{job.json()['id']}/links", json={"password": _PW})
diff --git a/backend/tests/test_upload.py b/backend/tests/test_upload.py
index d1e8c9c2..4e983b5a 100644
--- a/backend/tests/test_upload.py
+++ b/backend/tests/test_upload.py
@@ -123,15 +123,106 @@ def test_ingest_rejects_over_footprint_cap_before_writing(db, make_user, _root,
     assert db.query(MediaAsset).count() == 0  # and no orphan asset row
 
 
+# --- route: the auth-first body (P1) --------------------------------------------------------
+
+
+def _counting_body(chunks: int = 4, size: int = 1024):
+    """A request body that records how much of itself the server actually pulled.
+
+    httpx's ASGI transport feeds the app lazily, one `http.request` message per yielded chunk, so a
+    chunk that was never yielded is a chunk the app never asked for. That is the whole assertion
+    below: it is not "the upload was refused", it is "the refusal happened before the bytes moved".
+    """
+    pulled: list[int] = []
+
+    def body():
+        for _ in range(chunks):
+            pulled.append(size)
+            yield b"x" * size
+
+    return body(), pulled
+
+
+_BOUNDARY = "p1boundary"
+
+
+def _counting_multipart(chunks: int = 4, size: int = 1024):
+    """The same counter, wrapped in a real multipart envelope.
+
+    The content-type is the load-bearing part. Starlette's `request.form()` returns an EMPTY
+    FormData without touching the stream when the content-type is not multipart/urlencoded — so a
+    raw-body probe cannot tell "the route stopped reading strangers" apart from "there was nothing
+    here to parse". Only a well-formed multipart body makes the old shape actually spool.
+    """
+    pulled: list[int] = []
+    head = (
+        f"--{_BOUNDARY}\r\n"
+        'Content-Disposition: form-data; name="file"; filename="clip.mp4"\r\n'
+        "Content-Type: application/octet-stream\r\n\r\n"
+    ).encode()
+    tail = f"\r\n--{_BOUNDARY}--\r\n".encode()
+
+    def body():
+        pulled.append(len(head))
+        yield head
+        for _ in range(chunks):
+            pulled.append(size)
+            yield b"x" * size
+        pulled.append(len(tail))
+        yield tail
+
+    return body(), pulled
+
+
+def test_an_anonymous_upload_is_refused_before_a_byte_of_it_is_read(client):
+    """P1 — the defect E27 bounded but did not close.
+
+    While the route declared `file: UploadFile = File(...)`, FastAPI parsed and spooled the body
+    BEFORE the first dependency ran, so `require_human` could not gate it: an unauthenticated
+    caller could make the server write the whole per-file allowance (500 MiB by default) to temp,
+    per request. SEC-20's ceiling capped the number; it could not stop the writing.
+
+    The route now reads its own body from a dependency declared AFTER the auth one, the shape
+    `POST /api/debug-sessions/{sid}/screenshots` already uses. The claim under test is the strong
+    one — a stranger's bytes are never read WHATEVER they claim to send, so the probe sends the
+    multipart the old route would have spooled, not the raw body the new one expects."""
+    body, pulled = _counting_multipart()
+    r = client.post(
+        "/api/downloads/upload",
+        content=body,
+        headers={"content-type": f"multipart/form-data; boundary={_BOUNDARY}"},
+    )
+    assert r.status_code == 401
+    assert pulled == [], f"the server read {sum(pulled)} bytes from an anonymous caller"
+
+
+def test_an_authenticated_upload_still_has_its_body_read(as_user, make_user, _root, monkeypatch):
+    """The mutation check for the test above: a route that read NOBODY's body would pass it just as
+    well as the fixed one. The same counting body, signed in, must be pulled in full."""
+    _probe(monkeypatch, (None, None, None, None, None))
+    body, pulled = _counting_body()
+    c = as_user(make_user())
+    r = c.post(
+        "/api/downloads/upload",
+        content=body,
+        headers={"content-type": "application/octet-stream", "x-upload-filename": "clip.bin"},
+    )
+    assert r.status_code == 200, r.text
+    assert sum(pulled) == 4096
+
+
 # --- route ----------------------------------------------------------------------------------
 
 
-def test_upload_route_generic_forces_download(as_user, make_user, _root, monkeypatch):
+def test_upload_route_generic_forces_download(
+    as_user, make_user, _root, monkeypatch, upload_headers
+):
     _probe(monkeypatch, (None, None, None, None, None))
     c = as_user(make_user())
     r = c.post(
         "/api/downloads/upload",
-        files={"file": ("report.html", b"<script>alert(1)</script>", "text/html")},
+        content=b"<script>alert(1)</script>",
+        headers=upload_headers("report.html"),
     )
     assert r.status_code == 200, r.text
     job = r.json()
@@ -145,7 +236,7 @@ def test_upload_route_generic_forces_download(as_user, make_user, _root, monkeyp
 
 
 def test_upload_route_media_html_extension_not_served_as_html(
-    as_user, make_user, _root, monkeypatch
+    as_user, make_user, _root, monkeypatch, upload_headers
 ):
     """The XSS guard: a MEDIA-classified upload (real codecs) named with an executable extension must
     NOT be served with a guessed text/html type. The served Content-Type comes from the verified
@@ -156,7 +247,8 @@ def test_upload_route_media_html_extension_not_served_as_html(
     c = as_user(make_user())
     r = c.post(
         "/api/downloads/upload",
-        files={"file": ("evil.html", b"<script>alert(1)</script>", "text/html")},
+        content=b"<script>alert(1)</script>",
+        headers=upload_headers("evil.html"),
     )
     assert r.status_code == 200, r.text
     job = r.json()
@@ -177,7 +269,9 @@ def test_upload_route_media_html_extension_not_served_as_html(
     assert "attachment" in pr.headers["content-disposition"].lower()
 
 
-def test_public_watch_file_forces_download_for_generic_file(as_user, make_user, _root, monkeypatch):
+def test_public_watch_file_forces_download_for_generic_file(
+    as_user, make_user, _root, monkeypatch, upload_headers
+):
     """The login-free public serve of a generic upload (media_kind='file') must be a neutral
     force-download attachment — the branch that stops uploaded HTML/SVG executing on the public
     origin. This is a SEPARATE code path from the authenticated /file route, so it needs its own
@@ -186,7 +280,8 @@ def test_public_watch_file_forces_download_for_generic_file(as_user, make_user,
     c = as_user(make_user())
     job = c.post(
         "/api/downloads/upload",
-        files={"file": ("notes.html", b"<script>alert(1)</script>", "text/html")},
+        content=b"<script>alert(1)</script>",
+        headers=upload_headers("notes.html"),
     ).json()
     assert job["asset"]["media_kind"] == "file"
     link = c.post(f"/api/downloads/{job['id']}/links", json={"allow_download": True}).json()
@@ -197,7 +292,7 @@ def test_public_watch_file_forces_download_for_generic_file(as_user, make_user,
 
 
 def test_public_watch_file_serves_media_inline_with_real_type(
-    as_user, make_user, _root, monkeypatch
+    as_user, make_user, _root, monkeypatch, upload_headers
 ):
     """A recognised media container IS still served inline with its real audio/video type (so it
     plays in the watch page) — the fix must not break legitimate playback. A stream-only link →
@@ -206,7 +301,8 @@ def test_public_watch_file_serves_media_inline_with_real_type(
     c = as_user(make_user())
     job = c.post(
         "/api/downloads/upload",
-        files={"file": ("clip.mp4", b"fake-mp4-bytes", "video/mp4")},
+        content=b"fake-mp4-bytes",
+        headers=upload_headers("clip.mp4"),
     ).json()
     link = c.post(f"/api/downloads/{job['id']}/links", json={"allow_download": False}).json()
     pr = c.get(f"/api/public/watch/{link['token']}/file")
@@ -215,7 +311,9 @@ def test_public_watch_file_serves_media_inline_with_real_type(
     assert "inline" in pr.headers["content-disposition"].lower()
 
 
-def test_upload_route_enforces_per_file_cap(as_user, make_user, db, _root, monkeypatch):
+def test_upload_route_enforces_per_file_cap(
+    as_user, make_user, db, _root, monkeypatch, upload_headers
+):
     from app import sysconfig
 
     sysconfig.set_value(db, "download_upload_max_bytes", 5)
@@ -223,47 +321,48 @@ def test_upload_route_enforces_per_file_cap(as_user, make_user, db, _root, monke
     c = as_user(make_user())
     r = c.post(
         "/api/downloads/upload",
-        files={"file": ("big.bin", b"way over the cap", "application/octet-stream")},
+        content=b"way over the cap",
+        headers=upload_headers("big.bin"),
     )
     assert r.status_code == 422
     assert r.json()["detail"]["code"] == "upload_too_large"
 
 
 def test_upload_cap_cannot_be_disabled_with_zero(db):
-    """download_upload_max_bytes floors at 1: a stored 0 would read falsy in the route's
-    `if cap and size > cap` guard and SILENTLY disable the per-file cap, so set_value must reject it.
-    Pins the min=1 invariant so a revert to min=0 can't pass the gate."""
+    """download_upload_max_bytes floors at 1, and set_value must enforce it.
+
+    The falsy-0 footgun this originally guarded is gone — the route's old `if cap and size > cap`
+    read a stored 0 as "no cap at all", while `capped_stream_to` compares unconditionally and a 0
+    would refuse everything instead. Refusing everything is the safe direction, but it is still
+    wrong, and the knob is also the number the admin page and the 422 `limit` advertise. Pins the
+    min=1 invariant so a revert to min=0 can't pass the gate."""
     from app import sysconfig
 
     with pytest.raises(ValueError):
         sysconfig.set_value(db, "download_upload_max_bytes", 0)
 
 
-def test_upload_route_rejects_empty_file(as_user, make_user, _root, monkeypatch):
+def test_upload_route_rejects_empty_file(as_user, make_user, _root, monkeypatch, upload_headers):
     c = as_user(make_user())
-    r = c.post(
-        "/api/downloads/upload", files={"file": ("empty.bin", b"", "application/octet-stream")}
-    )
+    r = c.post("/api/downloads/upload", content=b"", headers=upload_headers("empty.bin"))
     assert r.status_code == 400
     assert r.json()["detail"]["code"] == "upload_empty"
 
 
-def test_redownload_rejects_an_upload(as_user, make_user, _root, monkeypatch):
+def test_redownload_rejects_an_upload(as_user, make_user, _root, monkeypatch, upload_headers):
     _probe(monkeypatch, (None, None, None, None, None))
     c = as_user(make_user())
-    job = c.post(
-        "/api/downloads/upload", files={"file": ("a.bin", b"data", "application/octet-stream")}
-    ).json()
+    job = c.post("/api/downloads/upload", content=b"data", headers=upload_headers("a.bin")).json()
     r = c.post(f"/api/downloads/{job['id']}/redownload")
     assert r.status_code == 409
 
 
-def test_edit_rejects_a_generic_upload_source(as_user, make_user, _root, monkeypatch):
+def test_edit_rejects_a_generic_upload_source(
+    as_user, make_user, _root, monkeypatch, upload_headers
+):
     _probe(monkeypatch, (None, None, None, None, None))
     c = as_user(make_user())
-    job = c.post(
-        "/api/downloads/upload", files={"file": ("a.bin", b"data", "application/octet-stream")}
-    ).json()
+    job = c.post("/api/downloads/upload", content=b"data", headers=upload_headers("a.bin")).json()
     r = c.post(
         "/api/downloads/edit",
         json={"source_job_id": job["id"], "edit_spec": {"trim": {"start_s": 0, "end_s": 1}}},
diff --git a/frontend/src/lib/api/core.test.ts b/frontend/src/lib/api/core.test.ts
index 2f23ca1f..ff9bf1de 100644
--- a/frontend/src/lib/api/core.test.ts
+++ b/frontend/src/lib/api/core.test.ts
@@ -23,6 +23,7 @@ vi.mock("../activeAccount", () => ({
 }));
 
 import { setDebugSession } from "../log";
+import { downloadsApi } from "./downloads";
 import {
   beacon,
   req,
@@ -46,6 +47,7 @@ class FakeXHR {
   onerror: (() => void) | null = null;
   onabort: (() => void) | null = null;
   sent = false;
+  body: unknown = null;
   constructor() {
     FakeXHR.last = this;
   }
@@ -53,8 +55,9 @@ class FakeXHR {
   setRequestHeader(k: string, v: string) {
     this.headers[k] = v;
   }
-  send() {
+  send(body?: unknown) {
     this.sent = true;
+    this.body = body ?? null;
   }
   abort() {
     this.onabort?.();
@@ -384,3 +387,26 @@ describe("the debug session a tab records into", () => {
     expect(sentHeaders().has("X-Debug-Session")).toBe(false);
   });
 });
+
+// P1 — the upload's wire shape is a security boundary, not a detail. A multipart part is a DECLARED
+// body, which the server reads and spools BEFORE its auth dependency runs; the raw body is what lets
+// `require_human` resolve first. A revert to FormData here would silently re-open that, and the
+// backend's own test only sees the shape the client actually sends.
+describe("uploadDownload's wire shape", () => {
+  it("sends the File as the raw body, with the names as percent-encoded headers", () => {
+    const file = new File(["abc"], "nyaralás videó.mp4", { type: "video/mp4" });
+    void downloadsApi.uploadDownload(file, { displayName: "Nyaralás 2026" });
+    const x = FakeXHR.last;
+    expect(x.body).toBe(file);
+    expect(x.body).not.toBeInstanceOf(FormData);
+    expect(x.headers["X-Upload-Filename"]).toBe(encodeURIComponent("nyaralás videó.mp4"));
+    expect(x.headers["X-Upload-Display-Name"]).toBe(encodeURIComponent("Nyaralás 2026"));
+    // Content-Type is the browser's to set from the File — an explicit one here would be a guess.
+    expect(x.headers["Content-Type"]).toBeUndefined();
+  });
+
+  it("omits the display-name header when there is no display name", () => {
+    void downloadsApi.uploadDownload(new File(["abc"], "clip.mp4"));
+    expect(FakeXHR.last.headers["X-Upload-Display-Name"]).toBeUndefined();
+  });
+});
diff --git a/frontend/src/lib/api/core.ts b/frontend/src/lib/api/core.ts
index 5d8f69d3..f567802a 100644
--- a/frontend/src/lib/api/core.ts
+++ b/frontend/src/lib/api/core.ts
@@ -372,16 +372,27 @@ export async function req<T = unknown>(
   }
 }
 
-// Multipart upload with progress. `req` rides `fetch`, which can't report upload progress, so a
-// file upload uses XHR instead: the caller gets a 0–100% callback as the body streams and an
-// optional AbortSignal to cancel a large upload. Everything else mirrors `req` — the per-tab
-// account header, credentials, the structured-detail localization, the global error modal for
-// 4xx/5xx, and an `HttpError` throw — so callers handle failures exactly as they do a `req` call.
-// The browser sets the multipart Content-Type (with its boundary), so we deliberately don't.
+// Upload with progress. `req` rides `fetch`, which can't report upload progress, so a file upload
+// uses XHR instead: the caller gets a 0–100% callback as the body streams and an optional
+// AbortSignal to cancel a large upload. Everything else mirrors `req` — the per-tab account header,
+// credentials, the structured-detail localization, the global error modal for 4xx/5xx, and an
+// `HttpError` throw — so callers handle failures exactly as they do a `req` call.
+//
+// `body` is a `Blob`/`File` for a raw upload or `FormData` for a multipart one. The raw shape is
+// what `/api/downloads/upload` uses since P1, and it is not a preference: a multipart part is a
+// DECLARED body, which FastAPI spools before the route's auth dependency runs. `cfg.headers` is how
+// such a caller sends the metadata that used to be sibling form fields.
+//
+// Content-Type is never set here. The browser derives it — the multipart boundary for FormData, the
+// Blob's own type otherwise — and overriding it would break the boundary.
 export function uploadReq<T = unknown>(
   url: string,
-  form: FormData,
-  cfg: { onProgress?: (pct: number) => void; signal?: AbortSignal } = {},
+  body: Blob | FormData,
+  cfg: {
+    onProgress?: (pct: number) => void;
+    signal?: AbortSignal;
+    headers?: Record<string, string>;
+  } = {},
 ): Promise<T> {
   return new Promise<T>((resolve, reject) => {
     const xhr = new XMLHttpRequest();
@@ -390,7 +401,7 @@ export function uploadReq<T = unknown>(
     const requestId = newRequestId();
     const started = performance.now();
     xhr.setRequestHeader(REQUEST_ID_HEADER, requestId);
-    for (const [name, value] of Object.entries(identityHeaders()))
+    for (const [name, value] of Object.entries({ ...identityHeaders(), ...cfg.headers }))
       xhr.setRequestHeader(name, value);
     if (cfg.onProgress) {
       xhr.upload.onprogress = (e) => {
@@ -443,6 +454,6 @@ export function uploadReq<T = unknown>(
       logRequest(requestId, "POST", url, started, { failed: "aborted" });
       reject(new HttpError(0, ABORTED));
     };
-    xhr.send(form);
+    xhr.send(body);
   });
 }
diff --git a/frontend/src/lib/api/downloads.ts b/frontend/src/lib/api/downloads.ts
index 1e07e265..f0f8584e 100644
--- a/frontend/src/lib/api/downloads.ts
+++ b/frontend/src/lib/api/downloads.ts
@@ -187,16 +187,23 @@ export const downloadsApi = {
   // Ingest a local file into the library (E7 S5). Multipart via XHR so the dialog can show upload
   // progress; any file type is accepted (media gets the full function set, a generic file is
   // download-only). Returns the already-`done` job the ingest creates.
+  // The body IS the file, and the two names ride as headers. Not a style choice: a multipart part is
+  // a DECLARED body, which FastAPI reads and spools BEFORE the route's `require_human` dependency
+  // runs — so with the old shape an unauthenticated caller could make the server write the whole
+  // per-file allowance to temp (P1). Headers are latin-1 on the wire, hence `encodeURIComponent`;
+  // the server percent-decodes both. Content-Type is left to the browser (the File's own type).
   uploadDownload: (
     file: File,
     opts: { displayName?: string; onProgress?: (pct: number) => void; signal?: AbortSignal } = {},
   ): Promise<DownloadJob> => {
-    const form = new FormData();
-    form.append("file", file);
-    if (opts.displayName) form.append("display_name", opts.displayName);
-    return uploadReq<DownloadJob>("/api/downloads/upload", form, {
+    const headers: Record<string, string> = {
+      "X-Upload-Filename": encodeURIComponent(file.name || "upload"),
+    };
+    if (opts.displayName) headers["X-Upload-Display-Name"] = encodeURIComponent(opts.displayName);
+    return uploadReq<DownloadJob>("/api/downloads/upload", file, {
       onProgress: opts.onProgress,
       signal: opts.signal,
+      headers,
     });
   },
   enqueueEdit: (body: {