hacking the code

Hi,
	I just download the Jigsaw_2.0beta1.zip file, and can run it. It
is really nice, now what I want is add a litter extendsion to the httpd:

GET /filename.htm http/1.0

After get the filename of request in server, I need to check the name of 
"filename.htm", if it is in my predefined format, I need to do some
extract things. Then the output filename may be different as this one.

Now when I look at the code, it is so complicate that I don't know where
to begin. Can someone tell me for this simple modification, What is the
best way?

Thanks in advance.
David Chan