POST /v1/files/upload exists so a client sends a file once and then names it:
inlining a 40 MB clip in every request is the waste it was built to remove. But
it stored its content-addressed files in the output directory and answered 501
when there was none configured -- so on such an install the only way to send
media was the inlining the endpoint replaces, and a client that had already
switched to uploading saw its photo edits fail outright.

An install always has a cache directory, so uploads fall back to it
(media.uploads_dir()). With an output directory nothing moves: an upload still
lands there beside the generated files. Both stores stay in the lookup path, so
an install that gains --file-path later does not lose track of what it already
holds, and /v1/files/<name> serves from the cache store only for up- names --
the cache directory does not quietly become a second public output directory.
501 now means there is nowhere at all to write; a non-writable store answers
507 rather than a 500 out of makedirs.

Composing a new video still needs an output directory, since there has to be
somewhere to write the result.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S8LAP3ueNzQJtwzhgVbq66