Here are some of the headers of a mail that both gnus and metamail
failed to grok as the sender intended them to, in the form I received
it.  It was apparently an animated gif, before microsoft outlook,
majordomo, and whatever other mail-manglers mangled it got through
with it.  I can forward the full message if anyone is interested in
looking at it.  

I've had this problem before, and I'm not sure it's a gnus problem.
As maintainer of a majordomo mailing list, I get sent messages which
bounce because they exceed a (fairly small) length limit.  I could
send them on, but I like to look at them before I do that, and I never
seem to be able to do that.

When I get the message, I see text that looks to me like mime headers, 
but that doesn't seem to buttonize like mime headers in gnus, or to be 
decoded into separate parts by "metamail -w".  I never see anything
resembling a gif.