>
cgi-bin/wwwthreads/showpost.pl?Board=mameworldarcadepcjamma&Number=2024&page=2...
> 
> How can you possibly attribute a MIME type to that??
Well in the case above, the forum is running from a Perl script: showpost.pl

You should try adding another MIME type of 
pl = text/html

Another good idea, if you've not done so already, would be to exclude
"unnecessary" pages. Often on web forums there are pages for sending emails to
members, members information, post message pages, etc.  A lot of this can be
excluding with filters, just be identifying the specific pages.