`ArcanistChmodLinter` should not allow certain MIME types to be executable

Summary: Fixes T5466. An image is an example of a binary which should //not// be executable. Modify the `ArcanistChmodLinter` to disallow certain blacklisted MIME types from being executable.

Test Plan: Created an executable image file and ran `arc lint` over this file.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: richardvanvelzen, epriestley, Korvin

Maniphest Tasks: T5466

Differential Revision: https://secure.phabricator.com/D9723