[api/attachments] Properly set the 'is_embedded' and 'message_id' parameters for the files endpoint when it is an attachment.

Summary:
Properly set the 'is_embedded' and 'message_id' parameters for the files endpoint when it is an attachment.
Fixes T473

Test Plan: regression

Reviewers: emfree

Reviewed By: emfree

Maniphest Tasks: T473

Differential Revision: https://review.inboxapp.com/D493