I recently upgraded from Visual Studio 2010 to the Visual Studio 2012 RC. The installer also installs IIS 8 Express which Visual Studio now uses as the default web server.>
I am using an ashx file in an MVC application and none of the above answers worked for me. IIS 10.
Here's what did work. Instead of changing "ExtensionlessUrl-Integrated-4.0" in IIS or web.config I changed "SimpleHandlerFactory-Integrated-4.0" for "*.ashx" files: