fix:custom file extension not support

[leslie2046]

Summary

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Close issue syntax: Fixes #<issue number> or Resolves #<issue number>, see documentation for more details.

fix #10637

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before:

after:

Checklist

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• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (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.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

[crazywoola]

Please link the existing issues as well.

It seems related to #10637, if this pr resolves this issue, please update the description above.

[melcurve]

Please link the existing issues as well.

It seems related to #10637, if this pr resolves this issue, please update the description above.

#10637 still not resolved. And #10640 seems right, don't know why it wasn't merged.