OK, so how do I *use* MIME?

[Hrvoje Niksic <hniksic@srce.hr>]

So now we have MIME handling in Gnus, and it should be possible to
program handling of new data types.  To take an example, I am
receiving patches in an ugly `application/x-gzip64' attachment, which
I want decoded by Gnus (please don't explain how broken that format is 
-- it's only an example.)

So I tried this in .gnus:

    (eval-after-load "mm-decode"
      '(progn
	 (push '("application/x-gunzip" inline-gunzip t)
	       mm-inline-media-tests)
	 (push '("application/x-gunzip" . inline) mm-user-display-methods)))

    (defun inline-gunzip (handle)
      (let ((b (point))
	    (text (mm-get-part handle)))
	(insert text)
	(save-restriction
	  (narrow-to-region b (point))
	  (call-process-region (point-min) (point-max) "gunzip" t t nil)
	  (mm-handle-set-undisplayer
	   handle
	   `(lambda ()
	      (let (buffer-read-only)
		(delete-region ,(point-min-marker) ,(point-max-marker))))))))

And indeed, it is fine, with two exceptions:

1) It does allow me to view the article in the buffer, but saving it
   or piping it to a command still saves the original junk, instead of 
   the decoded stuff.  Is there a way to hook into the way *contents*
   are handled, in order to allow easy saving of the junk?

2) Hooking into `mm-inline-media-tests' and `mm-user-display-methods'
   seems unclean, given that these variables are all but undocumented
   (hi Lloyd!).  How about having a variable for "user" handles, which 
   would be easy to access and modify?