Files & Uploads filtering (DOC-1316)

@martynjames @marcotuts @mhoeber @srpearce @lamagnifica please review
This is an initial draft. Still need to replace outdated screenshots; waiting for finalized UI.
Users can upload more than one file at a time, and they can also upload folders, correct? I think we should make that clear. Maybe "select the folder, file, or files that you want to upload"?