Ensure Filedropper.accepted_filetypes supports most extension types

[maximlt]

Fixes #7153

Filepond's acceptedFileTypes doesn't yet support file extensions (see pqina/filepond-plugin-file-validate-type#13) while Panel documents that they are accepted. This PR adds a hard-coded mapping from extension to MIME type (partially LLM-generated) that should cover most cases.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.10%. Comparing base (798a974) to head (6c2add5).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8380      +/-   ##
==========================================
+ Coverage   86.09%   86.10%   +0.01%     
==========================================
  Files         349      349              
  Lines       54734    54734              
==========================================
+ Hits        47121    47128       +7     
+ Misses       7613     7606       -7     

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[philippjfr]

Thank you!

[holovizbot]

This pull request has been mentioned on HoloViz Discourse. There might be relevant details there:

https://discourse.holoviz.org/t/file-drop-widget-not-working-as-expected/9057/6

[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.