OK, I found it in an old e-mail I once sent to the German distibutor Haage who also didn't seem to know about that feature 
It's a matter of just registering the MIME file types for .mht and .eml as
multipart/related
not as "message/rfc822" as they are incorrectly designated in Dopus as a standard.
Recapitulating, the .mht and .eml preview is back
Cheers, David.P
(broken image link)