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

Change-Id: Ia184943b340ad14666ec454cae52c0d87cfd8c84