8
It should work, if you put that content-type header in the right place, and also use HTML tags in the mail template.
If you view the message source of the received email in your email client, does that content-type header show up?
Also, it's possible that more headers are needed. You might look at the headers of another received HTML email, and see if any relevant additional ones are there.
P.S. I hate HTML email. It's evil, and you shouldn't use it anyway.