re-use files if you forget to name a mock

Summary: good title

Test Plan: made a mock with no title and the files i had attached showed up again correctly. named the mock and it worked!

Reviewers: epriestley, chad

Reviewed By: chad

CC: aran, Korvin

Maniphest Tasks: T2642

Differential Revision: https://secure.phabricator.com/D5261