How Do I Define Helper Applications

Have attempted to do so unsuccessfully. I have put in the mime type and 
the filename extension, e.g., for a .wav file, and indicated I wanted to
be asked what to do with the file. Mozilla insists on loading the file
into a new navigator window.

So I removed the definition of the helper app. Since that is now an 
unknown file type, Mozilla should ask, right? Wrong. It still downloads
the file into a new window.

What am I doing wrong? If there is a cookbook for setting up helper
applications, I couldn't find it with google.

Running Moz 1.4 on Solaris 9 on a Sparc machine.

TIA

Rod