I was just browsing through an ASP.NET book that said about changing the ASP.NET runtime for a single web application to select every file extension with the Internet Information Server Admin configuration to choose the right version of the ISAPI DLL.
This is too much work for me. It's a lot easier using the command line:
> aspnet_regiis.exe -s W3SVC/1/ROOT/mydemoweb
Scott Watermasysk mentioned a better way instead of the command line: