qp.mail.send.sendmail() does not output a content-type header, unless
the extra_headers parameter is specified... and this even if the
separate parameter mime_type is specifically supplied. These current
lines of code:

     if extra_headers:
         email.set_extra_headers(
             extra_headers + [
             "Content-Type: %s; charset=%s" % (mime_type, charset)])

should probably be changed to something like, such that a content-type
header is output independently of any extra_headers specified:

     extra_headers = extra_headers or []
     extra_headers.append("Content-Type: %s; charset=%s" % (mime_type,
charset))
     email.set_extra_headers(extra_headers)