R1:8ca5581a9c17

Skip attaching 'inline' text attachments

Summary:
Mail clients can send messages where the body is represented as 'inline'
attachments. Don't treat any such text attachments as actual attachments.

Test Plan:
toulouse, can you verify this fixes the issue?

Reviewed By: toulouse
Reviewers: toulouse
CC: aran, toulouse, epriestley
Differential Revision: 441