Send caption with file

[bmarty]

Thanks to matrix-org/matrix-rust-sdk#4273, it is now possible to set a caption when sending a file.

Consequently:

• the media preview must be used when sending a file, currently this step is skipped
• the caption may be sent along with the file
• the caption of the file must be rendered in the timeline
• the caption of the file must be rendered in the last message in the room list

[pixlwave]

@bmarty I'm closing this as I appears it was handled in #3902 and it is the only remaining task in the epic, but obviously feel free to re-open if there is more to do on your side :)