Fix Firefox upload fatal

Summary:
Several problems:

• With fpm-warmup, 'PhabricatorAccessLog' is always loaded, even if it hasn't actually initialized. Use a global instead (barf). I'll fix this when I refactor index.php, hopefully soon.
• The 'POST' check isn't sufficient in Firefox for HTML5 uploads -- not 100% sure why, maybe it encodes post bodies differently? I added an additional 'file' requirement, and will add this param to GET on all file uploads in a future diff.

See discussion in D2381.

Test Plan: Uploaded files with Firefox via drag-and-drop without various mysterious errors.

Reviewers: vrana, btrahan, jungejason

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D2405