[automerger] Disallow attaching files from our own EmailAttachmentProvider. am: 44a5db2dc0 am: ec59cd4c39 am: 7764908d8d am: 29f9af1b0a am: 1f74cbabba am: 9a7c825a34 am: 4c824af722

Change-Id: Ie4383110b4954a51d9eca2e01454410912f613ea