Set email format for individual mailboxes

A patch for our sales guy, who didn't want to receive his voicemail in gsm format, I've created a way to specify an email attachment format per mailbox, such that our guys who are still on Windows can receive their voicemail in wav49 format, while those of us who have Linux desktops can continue to have voicemail arrive in gsm format. See bug 6961 for the patch.