chore: LOCAL_FILE also try to use remote_url as Prompt message

[hjlarry]

Summary

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

Because of the CSP, all the remote file upload as LOCAL_FILE . But for the LLM prompt message, we can try to use remote_url.

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Checklist

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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

[laipz8200]

LGTM