Ignore .css files when using URL Routing on Webforms

The Problem: I'm using classic ASP.NET (3.5 SP1) and just added URL Routing. Now the problem is, that my .css file is also being routed through my controller, and therefore isn't being loaded properly. Here's my global.asax
    void Application_Start(object sender, EventArgs e)
    {
        RegisterRoutes(RouteTable.Routes);
    }

    private void RegisterRoutes(RouteCollection Routes)
    {
        Route r = new Route("fragen/{sys}/{url}", new rFragen());
        Routes.Add(r);       
    }
And my "rFragen" class:
        string sys = requestContext.RouteData.GetRequiredString("sys");
    & ...