Upload not found
/api/upload/part or /complete (or the v1 equivalents) could not find a pending multipart session for that fileId (404).
Why this happens
Wrong id, upload already completed/canceled/expired, deleted pending row, or init never succeeded. Expired sessions more often return Upload session expired (410) — related but distinct.
Diagnose and fix
1. Member: start a new upload from the beginning (fresh init).
2. Do not reuse an old fileId after cancel, complete, or a long pause.
3. Admin: confirm the file / multipart row still exists if debugging a specific id.
4. If init itself fails, fix that error first — parts will always 404 without a session.