Hi, Kimmo!
I don't know how to explain this, but the message you sent me as CC has a Content-Type header field:
Content-Type: multipart/alternative; boundary="_000_a352a6421f0240e0bc795ac1c4747d3celisafi_"
which is correct, the content is provided as both text/plain and text/html in separate parts with that boundary delimiting the parts. However in the shunted message object the Content-Type field isn't present although somehow it recognizes the message body as multipart, and content-type as text/plain.
I don't understand how this can happen. I've checked all the places where Mailman manipulates headers destructively, and all of those that might touch Content-Type immediately replace it.
On the other hand the absence of a Content-Type field but the content type is recognized a text/plain is consistent with there being no Content-Type field in the message header when Mailman receives it. I don't know how that can happen, either. What mail client and MTA are you using?
Regards, Steve