Gnus uses rule variables to decide how to view a file. All these variables are of the form
(list '(regexp1 command2) '(regexp2 command2) ...)
This variable is consulted first when viewing files. If you wish to use,
sox to convert an .au sound file, you could
say something like:
(setq gnus-uu-user-view-rules (list '("\\\\.au$" "sox %s -t .aiff > /dev/audio")))
This variable is consulted if Gnus couldn’t make any matches from the user and default view rules.
This variable can be used to say what commands should be used to unpack archives.