Fix Custom Tool File Upload: Resolve Multiple Files Recognition and Multipart Boundary Issues

[JimHeo]

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1. cannot multiple files upload: Files variable in Custom tool cannot be recognized correctlly #11255 (comment)
2. missing boundary in multipart error: The custom tool for multipart/form-data requests encountered a 'Missing boundary in multipart' error #13684

• ref: https://stackoverflow.com/questions/39280438/fetch-missing-boundary-in-multipart-form-data-post

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

[JimHeo]

Hi, Just a friendly reminder regarding my PR that addresses the following issues:

1. cannot multiple files upload: Files variable in Custom tool cannot be recognized correctlly #11255 (comment)

• When adding the multiple file upload feature in the custom tool, the input is mistakenly interpreted as a String rather than an Array[File], causing the files not to be received. For example, consider this FastAPI snippet:

@app.post("/upload-images/")
async def upload_images(files: List[UploadFile] = File(...)):
    if not files:
        raise HTTPException(status_code=400, detail="No files uploaded.")
        
    for file in files:
        if not file.content_type.startswith("image/"):
            return {"success": False}
        await file.read()
        
    return {"success": True}

2. missing boundary in multipart error: The custom tool for multipart/form-data requests encountered a 'Missing boundary in multipart' error #13684

• To support both multi-file and single-file uploads, the multipart/form-data schema needs to include the appropriate boundary. This PR addresses the issue to prevent users from having to manually modify the header to something like multipart/form-data; boundary=----WebKitFormBoundarydzemBAPhdeDfTCfR, ensuring a more robust and error-free implementation.

I’d appreciate it if you could review the changes at your earliest convenience. Thanks for your time!
@crazywoola @iamjoel @Nov1c444