Improve multiple file attachment UI

Currently the file attachment UI shows a single spinner for the first attachment uploaded.

If uploading multiple attachments at the same time, the spinner will show until the first file is uploaded and will then continue spinning until all other files are done and will display the % of the next file, even though these upload asynchronously, so it may jump from 100% to 48%.

You can also upload one big attachment, accepting the native upload dialog and then decide you will upload a couple other attachments in the mean time because the first one is taking a while. The spinner will show the progress of the first big attachment and the other attachments will upload ahead of the progress spinner as they're smaller but their progress will be hidden.

I propose that we improve the progress feedback, giving each attachment its own progress indicator.