multipart/related; not multipart/mixed; RFC 2387; html, embedded png

Hello

2 weeks ago I wrote about org-mime-htmlize, which html
emails and converts latex formula to embedded png.
I attach the corresponding htmlized message below.

The resulting mail is displayed in a couple of email
The reason is that the message contains the line
 Content-type: multipart/mixed; boundary="=-=-="
in its header while thunderbird requires 
 Content-type: multipart/related; boundary="=-=-="
and the thunderbird developers insist that this is the 
RFC 2387 standard.

Could gnus offer this possibility or maybe it is already
there and I miss the relevant setting?
Thanks

Uwe Brauer 

    -  test-org-msg (org-mime-htmlized msg)

    -  test-org-mime.eml how the  file is send.

    -  Test-org-mime-related.eml how it is displayed
       correctly in thunderbird

From: Uwe Brauer <uwe_brauer <at> mat.ucm.es>
Subject: test-org-mime
Date: 2012-04-10 15:36:58 GMT

From: Uwe Brauer <uwe_brauer <at> mat.ucm.es>
Subject: test-org-mime
Date: 2012-04-10 15:36:58 GMT