The man page documentation for metamail says that:
metamail -w %s
will MIME decode the file, and then save it to a file, instead of sending
it through a viewer. metamail.1 (the man page) also telly you how to run
metamail on the (DOS) command line, in general. I am surpirsed you did not
receive metamail.1 when you obtained the program itself.
If you run win98, of course, you must start a DOS box to run metamail on
the command line. There are also some progmams that create a pseudo
commanding within windows to run both windows and dos programs, by
typing program names and switches on a command line.
Finally, I thought of a trick to make metamail save to a file optionally,
for certain files. Say you had a line like,
text/*; save.bat %s
in mailcap. Then, you could put prompts and ``type %1 > filename''
or whatever you wanted in save.bat, and metamail should run save.bat
for you whenever it encounters mime encoded text files.