fix: add PDF MIME fallback for uploads

[BboyGT]

Summary

• adds .pdf to the upload MIME fallback map as �pplication/pdf
• adds a regression test for PDF fallback behavior
• keeps unknown extensions falling back to ext/plain

Fixes #16485.

Testing

• Ran git diff --check
• Not run locally: dependencies are not installed in this fresh checkout

[BboyGT]

Happy to adjust the test or placement if you prefer a different convention. I’m also happy to keep helping with small upload/admin reliability fixes like this.