public ActionResult Downloadfile(string filename, string fileloaction, string extention)
        {
        ...
        }

$.ajax({
          type: 'POST',
          cache: false,
          url: 'xxx/downloadfile',
          data: { 'filename': filename, 'fileloaction': fulllink, 'extention': extention },
          success: function () {
             ...
            },//end success
           error: function (e) {
              console.log(e.statusText);
              }//end error
            });