Rename "Upload Image" dialog to "Upload File" to match purpose
Summary: Fixes T3940.
Test Plan: Clicked button, looked at dialog.
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley
CC: nmalcolm, Korvin, epriestley, aran
Maniphest Tasks: T3940
Differential Revision: https://secure.phabricator.com/D7287