fix: update workflow doc

[sorphwer]

Summary

The original file variable has been replaced with a file array, so the doc needs to be updated.

Takeaways:

1. The original documentation shows a files attribute that does not exist in the current version.
2. Move the format description inside inputs to make the documentation more straightforward.
3. Update the data and code examples to use a file array instead of a single file. Otherwise, users will encounter the following error when following the current documentation:

400, {"code": "invalid_param", "message": "XXX in input form must be a list of files", "status": 400}

Screenshots

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After

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