Allow drag-and-drop uploads from Chrome download shelf

Summary: Fixes T2811. Chrome requires this, but only for files dragged from the download shelf, I guess? Not entirely sure what's going on here. I couldn't find much documentation on this and figured it out mostly by process of elimination.

Test Plan: Drag and drop still works everywhere, plus from the Chrome download shelf.

Reviewers: blc, btrahan, skrul

Reviewed By: skrul

CC: aran

Maniphest Tasks: T2811

Differential Revision: https://secure.phabricator.com/D5528