Upload file, using prefix.

gistfile1.js

function uploadFile(files) {
  if (files && files.length > 0) {
    var file = files[0];
    var params = { Key: getPrefix() + file.name, ContentType: file.type, Body: file };
    bucket.putObject(params, function(err, resp) {
       updateFileList();
    });
  }
}