Support for documents in multi-modal models

[Areiser]

I've searched for this feature and found the following issue: #820

As far as I can see, there is only support to pass images to the model so far using ImagePart.

Is support for generic documents (like PDFS/xlsx/...) planned? We have a use case where you can pass generic Uploads to the model, where we supply the Mime type, similar to Spring AIs Media.