fix: create_blob_message of tool will always create image type file

[hjlarry]

Summary

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Now there are some tools can generate video/audio format of files, but the create_blob_message API of tool will always generate image file type, so the speech2text tool will validate the file type may raise error.

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• This change requires a documentation update, included: Dify Document
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[laipz8200]

I’d prefer if you could make get_file_type_by_mimetype a private function within the file_factory module and use it in _build_from_tool_file whenever the type isn’t available. You can get the file’s mime-type from tool_file.mimetype.

After that, we can remove the need to pass in the type.