Our apache install was returning tgz and gz files as type text/plain, and this was due to a blank for type application/x-gzip. Fix was to add the line application/x-gzip gz tgz to /etc/mime.types
Fixed in CVS, will be in the next mailcap release (2.0.4-1)
Related to this, what is the correct entry for *.bz2 files?