Trim "\n" from mimeType field in phabricator_file.file

Summary: See patch comment, which explains the situation.

Test Plan: Ran upgrade, verified mimeType field is \n-free in database.

Reviewers: davidreuss, btrahan

Reviewed By: davidreuss

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1624