R1:549146bc7cc4

Move ALL files to serve from the alternate file domain, not just files without
"Content-Disposition: attachment"

Summary:
We currently serve some files off the primary domain (with "Content-Disposition:
attachment" + a CSRF check) and some files off the alternate domain (without
either).

This is not sufficient, because some UAs (like the iPad) ignore
"Content-Disposition: attachment". So there's an attack that goes like this:

- Alice uploads xss.html
- Alice says to Bob "hey download this file on your…