This and subsequent patches will eliminate the need for typed
attachments.

BUG=angle:732

Change-Id: Ia1cfa672855b577f7a2ce219e6ed8ca1e3e16cc1
Reviewed-on: https://chromium-review.googlesource.com/213858Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>