JavsScript: get disposition header for response

getDispositionHeader.js

function(userAgent, fileName) {
  var suffix = '.zip';
  if(userAgent.indexOf('msie') >= 0 || userAgent.indexOf('chrome') >= 0) {
      return 'attachment; filename=' + encodeURIComponent(fileName) + suffix;
  } else if(userAgent.indexOf('firefox') >= 0) {
      return 'attachment; filename*="utf8\'\'' + encodeURIComponent(fileName)+'"' + suffix;
  } else {
      return 'attachment; filename=' + new Buffer(fileName).toString('binary') + suffix;
  }
}