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 Still need to replace outdated screenshots; waiting for finalized UI.
That's true, but I think our documentation should reflect that it *is* possible to upload more than one file at a time. People who only see the UI won't realize that's possible and might spend a long time uploading individual files. :-(