The following works in IIS 6 and can probably be adapted for other web servers.

The problem with your example is that the URL is not handled by asp.net at all because the content type is not one of those specified as dynamic. To alter this behaviour open the IIS management tool and go to the DNN web site, right click and choose properties. From the home directory choose configuration and then choose insert to add a wildcard mapping in the bottom box. The mapping needs to point to the asp.net dll for the version of asp.net that you are running. The easiest thing to do is to look at the top box for the .aspx mapping, choose edit and copy the path from there.

The downside to this is that now all content will go via asp.net which leads to a small performance hit for static contant such as image files.