Asterisk voicemail messages are generated from tmp
    
    When asterisk creates a voicemail message to send to the end user, the message
    recording is a temporary file (labeled asterisk_tmp_t) which is then picked up
    by the system mailer. Hence, we need to mark asterisk_tmp_t as
    mta_system_content().
    
    Signed-off-by: Sven Vermeulen <sven.vermeulen@siphos.be>