[automerger] Disallow attaching files from our own EmailAttachmentProvider. am: 44a5db2dc0 am: ec59cd4c39 am: 7764908d8d am: 29f9af1b0a am: 1f74cbabba am: 9a7c825a34 am: 4c824af722 am: 425e9f9156 am: c94203d0c3 am: 5d2c780a76 am: 35ead32a64
am: f842517566

Change-Id: I4a71dd653951e4a500244388d86a7ac07a04bbe2