[exim] check for no Content-Type header?

Hi,

I installed a RAID monitor software and it sent some notification
emails through my mail server that had no Content-Type header so they
got stuck on the mail queue.
I've added this check for non Content-Type in a system-filter.mf file
but is there a better way?

Thanks,

Oliver

# Exim filter

if $message_body does not contains "Content-Type:"
then
logfile /var/log/exim4/no_content_type_log.txt
logwrite "$tod_log:$message_id:no content-type"
headers add "Content-Type: text/plain"
endif