If I understand the KMimeMagic code properly, then what you are trying to accomplish is possible by putting an extra *.magic file in the /etc/kde3/magic directory. It should be used automagically by the KDE MIME system. If you need an example, install the kolf package, it contains such a file iirc.
Please let us know if this fixes your problem, and if this bug can be closed. cheers domi
