[Route("{type}/{library}/{version}/{file?}/{renew?}")]
public ActionResult Index(EFileType type, 
                          string library, 
                          string version, 
                          string file = null, 
                          ECacheType renew = ECacheType.cache)
{
 // code...
}

https://stackoverflow.com/questions/24678045/routing-optional-parameters-in-asp-net-mvc-5