Execute Java program from file extension

Hello,

Hmmm, I have a plumbing problem - and it is difficult to explain.
I need to connect a Java program on the client to some server XML codes.

Basically, I've been given a Java program to execute on client side.
When the user hits the "Download" button, the server creates an XML file
and send it to the client as a file with the file extension ".rcm".
This file extension has already been registered via an install program on
the client.
It will then spawn/execute the client Java program.

I'm not sure what is the server-side response that I have to send to the
client before the actual data ( that is the ".rcm" file).

Is there some magic HTTP header that I send that can get this to work?

Here is a stab at what I thought would do it:

Content-type: application/octet-stream
'Content-Length: '. filesize(4096)

Content-Disposition: attachment; filename='MYFILE.RCM"


Thanks in advance for you help.

Regards,
bruce