IIS, by default, prohibits service of many file extensions, .cs is one.
So the request is being killed by IIS ASP.net before it hits the REST handlers.
This can be fixed fairly easily.
%SYSTEM%\Microsoft.NET\Framework\v2.0.50727\CONFIG\Web.Config
....
<httpHandlers>
<add path="*.cs" verb="*" type="System.Web.HttpForbiddenHandler" validate="True"/>
....
And of course this is just my supposition. I am not privy to the inner workings of the SO servers.