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 Still need to replace outdated screenshots; waiting for finalized UI.
I can never tell for sure when I'm supposed to bold the names of things or leave them unbolded, but it seems to me that Files & Uploads should be bolded here for consistency with the top of the page. But I'm probably confused...!