[automerger] Disallow attaching files from our own EmailAttachmentProvider. am: 44a5db2dc0 am: ec59cd4c39 am: 7764908d8d am: 29f9af1b0a am: 1f74cbabba am: 9a7c825a34 am: 4c824af722 am: 425e9f9156

Change-Id: If964a46ed388046df56d971c3a51ec59ea099aea