fix:Improve MIME type detection for remote URL uploads using python-magic

[yjc980121]

Resolved issue where binary files (e.g., .docx) were misidentified as .bin due to defaulting to application/octet-stream. Added python-magic as a secondary fallback to accurately detect MIME types from file content for remote URL uploads.

Summary

Resolved issue where binary files (e.g., .docx) were misidentified as .bin due to defaulting to application/octet-stream. Added python-magic as a secondary fallback to accurately detect MIME types from file content for remote URL uploads.

Fixes #12551

Dependency Check

• python-magic is implicitly available as a transitive dependency (via another package), so no additional changes to pyproject.toml or poetry.lock are required.

Checklist

[!IMPORTANT]

• 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 ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

[laipz8200]

Hi @yjc980121, thank you for your contribution! Please run dev/reformat to pass the CI check.

[laipz8200]

Hi, some conflicts need to be resolved.