fix: allow fallback to remote_url when url is not provided

[fujita-h]

Summary

When handling files from a URL, the parameter should be remote_url, so this function should also get it from remote_url. However, the actual implementation references the url parameter.
I was unable to confirm whether the url parameter was actually used, so the original parameter implementation has been left intact.
If the url parameter is not used, you can delete this part.

Fixes #12454
Also Fixes #12209

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