var fileReference:FileReference;

private function chooseFile():void {
  fileReference = new FileReference();
  fileReference.addEventListener(Event.SELECT, function(e:Event):void {
    uploadFile();
  });
  fileReference.browse();
}

private function uploadFile():void {
  trackApi.uploadFileReference({track: fileReference}, {
    onResponse: function(track:Object):void {
      trace('id: ' + track.id + ', md5: ' + track.md5);  // id: TRMVA0211BC6E08329, md5: 2f45abacba9e9d2312afa63a8df10d23
    },
    onEchoNestError: function(error:EchoNestError):void {
      trace(error.code + ': ' + error.description);
    }
  });
}