Policy - lock down file loading in mail reply handler path

Summary: Ref T7094. This one is really straight-forward since $this->actor is always populated and the right thing to do here.

Test Plan: used the ole thinking noodle since testing email w/ attachments is really hard

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7094

Differential Revision: https://secure.phabricator.com/D11589