Fix attachment behavior when using IMAP storage for voicemails

1. Filenames had an extra "msg" in the attachment name
2. The attachment was being saved twice

(closes issue #12894)
Reported by: jaroth
Patches:
      imap_attach.patch uploaded by jaroth (license 50)



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@124180 65c4cc65-6c06-0410-ace0-fbb531ad65f3