fix: cannot upload animated webp image as app icon

[xuzuodong]

Summary

1. Fixes cannot upload animated webp image as app icon (described in feat: support png, gif, webp #7947 (comment))
2. Fixes type errors
3. Minor function refactor for better readability

Screenshots

Before	After
2024-10-08.09.35.41.mov	2024-12-07.22.41.29.mov

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