按照koa官方给的例子做文件上传失败，用koa做过文件上传的同学能传授下经验吗？

按照官方给的这个例子去做https://github.com/koajs/examples/blob/master/upload/index.js 可是总是获取不到上传的内容，进入不到while里面：

app.use(function *(next){
  // ignore non-POSTs
  if ('POST' != this.method) return yield next;

  // multipart upload
  var parts = parse(this);
  var part;

  while (part = yield parts) {
    var stream = fs.createWriteStream('/tmp/' + Math.random());
    part.pipe(stream);
    console.log('uploading %s -> %s', part.filename, stream.path);
  }

  this.redirect('/');
});