如何在codeigniter中一张表单上传多个文件类型？

<form method="post" action="upload_controller/upload_files" enctype="multipart/form-data">
    <input type="file" name="userfiles[]" multiple>
    <input type="submit" value="Upload">
</form>

class Upload_controller extends CI_Controller {
    public function upload_files() {
        $config['upload_path'] = './uploads/';
        $config['allowed_types'] = 'gif|jpg|png';
        $config['max_size'] = 1024;

        $this->load->library('upload', $config);

        if (!$this->upload->do_upload('userfiles')) {
            $error = array('error' => $this->upload->display_errors());
            // 处理上传错误
        } else {
            $data = array('upload_data' => $this->upload->data());
            // 处理上传成功
        }
    }
}