Metamail decides what to do with a message part based on the preliminary
info. about this part in the header (e.g., the suffix of the filename),
and the mailcap file that tells it what programs to run for what files.
If you dont have the right information, or filename for a file in the
message header and/or you dont have the right program defined for that
type of file -- trouble is bound to ensue.
Alas, metamail usually only gives you the option of saving a file to
disk if it gets stumped, and does not know how to type the file. Then
it prompts for saving to disk. You can force this behaviour for eavery
type by using the -w option (e.g., use metamail -w). Once on disk,
you can run whatever programs you want.
I would like the option of saving a file (e.g., and executable) to
disk each time, even if I want it displayed on the screen too.
There are some programs (e.g., gdv) that claim to be able to interpret
and display about 400 types of images and text formats. In theory, you
could use these programs for virtually all the message types.