Fix an issue with FileMailReceiver not working well

Summary: Ref T7199. Guess no one has ever tried to reply to file mail.

Test Plan: Used bin/mail receive-test to send mail to files.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7199

Differential Revision: https://secure.phabricator.com/D12228