R1:355b753df70c

Prevent file download without POST + CSRF

Summary: This prevents <applet /> attacks unless the attacker can upload an
applet which has a viewable MIME type as detected by `file`. I'm not sure if
this is possible or not. It should, at least, narrow the attack window. There
are no real tradeoffs here, this is probably a strictly better application
behavior regardless of the security issues.
Test Plan:
- Tried to download a file via GET, got redirected to info.
- Downloaded a file via POST + CSRF from…