var getFiletype = function(string){
  var lastPeriod = string.lastIndexOf('.') + 1, //char after .
      length = string.length,
      diff = length - lastPeriod;
  if (lastPeriod > 0 && diff <=5) {
    return string.substring(lastPeriod,length);
  }
};

var filetype = getFiletype(url);