I don't know if that's a typo but you have to remove the ".ex" in the name. It must be called just "postinst".

There is:

postinst
preinst
prerm
postrm

And a few others. The *.ex and *.EX's are created as examples by the dh_* series of tools. You are meant to modify and then rename them.

Aptitude, synaptic, apt-get, are all different wraps to using "dpkg" as the actual install command. Dpkg itself is what uses that file.