如何使用codeigniter-restserver上传文件？

<?php
require APPPATH . 'libraries/REST_Controller.php';

class Upload extends REST_Controller {

    public function __construct() {
        parent::__construct();
        $this->load->library('upload');
    }

    public function index_post() {
        // 设置文件上传配置
        $config['upload_path'] = './uploads/';
        $config['allowed_types'] = 'gif|jpg|png';
        $config['max_size'] = 2048; // 2MB
        $this->upload->initialize($config);

        if (!$this->upload->do_upload('file')) {
            // 文件上传失败
            $error = array('error' => $this->upload->display_errors());
            $this->response($error, REST_Controller::HTTP_BAD_REQUEST);
        } else {
            // 文件上传成功
            $data = array('upload_data' => $this->upload->data());
            $this->response($data, REST_Controller::HTTP_OK);
        }
    }
}

$route['upload'] = 'upload';