Receiving Multipart E-Mails with ActionMailer

I am writing an application that receives emails from different sources
and some are encoded as multipart/alternative. I want to store the html
and the plaintext parts separately but ActionMailer/TMail gives no
method to do this easily. After searching for a long time I have only
found this:
http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-list/38051
But I don’t know how to make it work for me. Does anybody have an idea
who to takle that?