ルートを登録するときは、次の無視ルールを追加してみてください。
public static void RegisterRoutes(RouteCollection routes)
{
/* Ignore static content, see
http://weblogs.asp.net/rashid/archive/2009/04/03/asp-net-mvc-best-practices-part-2.aspx
*/
routes.RouteExistingFiles = false;
routes.IgnoreRoute("Content/{*pathInfo}");
routes.IgnoreRoute("Scripts/{*pathInfo}");
routes.IgnoreRoute("Styles/{*pathInfo}");
routes.IgnoreRoute("{*favicon}",
new { favicon = @"(.*/)?favicon.([iI][cC][oO]|[gG][iI][fF])(/.*)?" });
//Ignore handlers and resources
routes.IgnoreRoute("{resource}.ashx/{*pathInfo}");
routes.IgnoreRoute("{resource}.axd/{*pathInfo}");
// your routes go here
}