chore: the consistency of MultiModalPromptMessageContent

[hjlarry]

Summary

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

Currently, when files are sent to various LLM, each model may require different parameters. For instance, some models expect:
{"image_url": url or base64 with mime_type}
Others might require separated mime_type and b64data, formatted as:
{"mime_type": "", "data": ""}
And some may utilize a parameter that specifies the format:
{"format": "..."}
To enhance the readability of the code and to maintain consistency across all MultiModalPromptMessageContent, it is proposed that we standardize these parameters.

Additionally, it is suggested to change the MULTIMODAL_SEND_IMAGE_FORMAT and MULTIMODAL_SEND_VIDEO_FORMAT to a unified MULTIMODAL_SEND_FORMAT, as it seems that a single environment variable would suffice for all purposes.

Screenshots

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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

[laipz8200]

LGTM