mb_send_mail + multipart e-mail - why is this so difficult?

Hello,
I am trying to send an e-mail in unicode format, so am using the
mb_send_mail() function. I have to use this function, I cannot use
mail().

Unfortunately, when I put a valid multipart message through
mb_send_mail() it base64 encodes it, giving me two Content-Type
headers.

I really don't belive I'm the first person to send a multipart e-mail
through mb_send_mail(), how does everyone else do this?

Thanks.

.