When an email reply to a task includes files, attach them to the task

Summary:
Allow files to be attached to a task by attaching them to an email reply to the
task.

Test Plan:
Applied this patch live since I haven't managed to get inbound email configured
locally, then attached files to a task via email.

Reviewed By: jungejason
Reviewers: tuomaspelkonen, jungejason, aran
CC: anjali, aran, jungejason
Differential Revision: 369