Added + icon for File Application

Summary:
I personally upload plenty of files on my own, so I added it to the File application, too

thanks for adding the others btw, I love them

Test Plan: saw it pop up on the home page

Reviewers: chad, epriestley, btrahan, codeblock

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4583