fix: When chatflow's file uploads changed from not being supported to…

[jiangbo721]

… supporting multiple different types, the backend changed to a bug that only supported images.

Summary

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.

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

Screenshots

Before	After
...	...

bug is: https://github.com/user-attachments/assets/9754ceeb-fa80-4b2a-be50-3a0460ee7a3c

Checklist

[!IMPORTANT]
Please review the checklist below before submitting your pull request.

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