handleProgress = function(e){
    if (e.total > 0) {
      e.percent = e.loaded / e.total * 100;
    }
    e.direction = 'download';
    self.emit('progress', e);
  }
POST `url` with optional `data` and callback `fn(res)`.

@param {String} url
@param {Mixed} [data]
@param {Function} [fn]
@return {Request}
@api public