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