On Desktop, I usually just drag and drop files into a comment box (or use arc upload). On mobile, I usually go to /file/upload/, which is annoying because it usually means typing in the URL.
One possible solution would be to add "File" tp the quick create menu. Another possible solution would be to add some sort of "Add attachment" button. (Actually, it seems that there is one, but clicking it just says "To add files, drag and drop them into the comment text area").