Support sending file attachments

[JKrag]

From a storage point of view, my plan is that the file(s) should exist in the specific commit only, not just simply added to a folder with all previous sent files. The cleanest way to achieve this is probably to add the file directly to the git index before doing the commit. But I guess it is unavoidable that the file will still be seen as a deletion in the next commit in a git log.

I am still uncertain about the UX of this. Sending them is probably easy enough, probably just using a slash command, but I still have to figure out a good UX for displaying and ultimately retrieving files at the receiving end.