Use Filesystem::getMimeType() instead of file

Summary: The file binary doesn't exist everywhere, use the more flexible
wrapper introduce in D1609.

Test Plan: Uploaded a file via drag-and-drop, it got MIME'd correctly.

Reviewers: btrahan, davidreuss, Koolvin

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T869

Differential Revision: https://secure.phabricator.com/D1615