Enhancement: Ability to upload multiple media files

It would be a good idea to allow users to upload (attach) multiple media files (photos and other support media files, if any) in one go.

Currently, one can upload only one file at a time. At least, this is the case on Android and Mac OS clients.

Start by locating the Android and macOS client upload flows and checking how a single attachment is selected and sent. Done means users can select and upload multiple photos or other supported media files in one action on both clients, with existing attachment behavior preserved.

Written by the indexing model from the issue text.