Gemini handling for image/gif needs improvement

[pwang347]

Gemini models do not support GIF images, see https://ai.google.dev/gemini-api/docs/image-understanding#supported-formats
We need to make sure that either we don't allow attaching it when Gemini is selected or to convert the image instance on upload to png. When transferring from another model, we also need to drop those image/gif references retroactively.

[isidorn]

@pwang347 any way to try to handle this as part of January? CAPI just raised this as happening often - I can get the data to see how often it really happens, just let me know

[vijayupadya]

@isidorn is the issue CAPI raised about the general image type required issue (err below) OR specifically GIF issue?

Request Failed: 400 {"error":{"message":"image media type is required","code":"invalid_request_body"}}

This above issue is already fixed last week.