fix:change download file ext

[marsDes]

change download file ext

Summary

can not open the download's message_file ,xxx.bin
to fix the filename use the right file extension

Tip

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

Screenshots

Before	After
bee68c5ae5424a4fbde46dbf040de854.bin	bee68c5ae5424a4fbde46dbf040de854.xlsx

Checklist

Important

Please review the checklist below before submitting your pull request.

• 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 an existing issue or open a new issue and link it in the description.