Multiple file upload captions

Is your feature request related to a problem? Please describe.
When selecting multiple files to upload, the user is presented with a modal displaying the file names and a caption field. When sending the files, each file is sent as its own message, and the caption is only applied to the last message. There is no way to know if the caption is meant for multiple files.

Describe the solution you'd like
Since multiple files are uploaded as multiple messages, I think it makes sense for each file to have its own caption field. The modal would be a scrolling list of files and caption fields. Bonus points if it shows a file preview rather than just the file name.

Describe alternatives you've considered

1. The caption could be applied to each individual message.
2. Multiple file uploads could be tagged so that the app knows to display them together. This would require support in all Talk apps.

Additional context
The current behavior matches the Nextcloud Talk web app. From a technical perspective though, there's nothing preventing individual captions, and it would work on all Talk apps, since each file is its own message.

Some thought would have to be given to how to handle draft messages (see #6053 )

Start with the Android multiple-file upload modal and trace how its caption is applied to the separate file messages. Review draft-message handling in relation to #6053 before deciding how each file's caption should be retained. Done means each uploaded file can have its own caption, with the behavior working for the app's individual messages; file previews are optional.