I wanted to use this gem to generate a multipart/related message sent over HTTP. Unfortunately, the HTTP server denied my requests complaining about the errorneous content type. I did some research and found that for multipart/related a root content type actually needs to be defined (see RFC 2387, Chapter 3.1), e.g.:
Content-Type: multipart/related; boundary="abcdefg"; type="application/xml"
It seems that not all implementations follow this specification, but my server does :).
So perhaps you could add this to your gem, e.g.: