koa multipart

handler.js

router.post('/', multipart, function *() {
  if (this.state.files && this.state.files.media) {
    this.state.files.media.pipe(fs.createWriteStream(/* output */));
    // stream error handling omitted
  }
});

multipart.js

module.exports = function *(next) {
  var parts = parse(this, { autoFields: true })
  ,   part;

  this.state.files = {};

  while (part = yield parts) {
    this.state.files[part.fieldname] = part;
    part.resume();
  }

  this.body = this.body || parts.field || {};

  yield next;
};