We never did resolve the question of how compression should be done in
MIME, for a number of very good reasons. This means there's no ideal
answer to your question.
If I were you, and if this is an application over which I had a fair
amount of control, I might consider using a content-type of
"application/x-compressed-postscript". You can then configure mail
readers to do the right thing with the data, given that they are
configured to understand this subtype. (This is trivial with metamail,
for example.)
There will doubtless be people who will protest very loudly that the
right way to do this is something more like a content-type of
"application/postscript; conversions=x-compress". I think that neither
approach is entirely satisfactory, but that the former is less likely to
bomb badly (e.g. by sending raw compressed postscript to a printer) on a
MIME-compliant system that knows nothing about compression. -- Nathaniel