Use "application/octet-stream" as the FileResponse media type fallback

[ATOM00blue]

Summary

FileResponse falls back to text/plain when the media type cannot be guessed from the filename or path:

if media_type is None:
    media_type = guess_type(filename or path)[0] or "text/plain"

For files with an unrecognized extension this means the response is served as text/plain, so browsers render the bytes inline as text instead of downloading the file. This was reported in discussion #3106 (an Outlook .msg file rendered as text rather than triggering a download).

application/octet-stream is the correct default for unknown content per RFC 2046 ("MIME implementations must at a minimum treat any unrecognized subtypes as being equivalent to application/octet-stream") and the MDN guidance. It's also what Django and Werkzeug use:

• Django: content_type = content_type or "application/octet-stream"
• Werkzeug send_file falls back to application/octet-stream

The earlier choice of text/plain (#1027) was made on the assumption that Django used text/plain, which isn't the case.

This affects FileResponse directly and StaticFiles, which serves files through FileResponse. Passing an explicit media_type is unchanged.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[Copilot]

Pull request overview

Updates FileResponse to use a safer default Content-Type when a file’s MIME type cannot be inferred, aligning behavior with common HTTP/MIME expectations and improving browser handling of unknown file types.

Changes:

• Change FileResponse fallback media type from text/plain to application/octet-stream when mimetypes.guess_type() cannot determine a type.
• Add a regression test asserting the new fallback Content-Type behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
starlette/responses.py	Updates FileResponse default media type fallback to application/octet-stream.
tests/test_responses.py	Adds a test covering the unknown-extension fallback Content-Type.

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[Kludex]

Makes sense.

Thanks. 🙏