Postscript attachments not shown in message preview

The application/postscript mime type had been handled by the text-hightlight
module, but thinking of it, it doesn't make much sense, because most users
would prefer to see the content rather in a postscript viewer, than
the source of the postscript file, thus this change removes the mime type
from the text-hightlight module. There had been also issues about thinking
that certain parts had been handled, while they did not, which this change
fixes as well.

The issue had been reported downstream at:
https://bugzilla.redhat.com/show_bug.cgi?id=1187327