Codeigniter:使用Ajax将文件发送到服务器时出现错误400

$.ajax({
    url: 'your_upload_controller/upload',
    type: 'POST',
    data: new FormData($('#upload_form')[0]),
    processData: false,
    contentType: false,
    success: function(response) {
        console.log('File uploaded successfully');
    },
    error: function(xhr, status, error) {
        console.error('Error uploading file: ' + error);
    }
});

<?php
defined('BASEPATH') OR exit('No direct script access allowed');

class Upload extends CI_Controller {

    public function __construct() {
        parent::__construct();
        $this->load->helper(array('form', 'url'));
    }

    public function upload() {
        $config['upload_path'] = './uploads/';
        $config['allowed_types'] = 'gif|jpg|png';
        $config['max_size'] = '2048';

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

        if (!$this->upload->do_upload('userfile')) {
            $error = array('error' => $this->upload->display_errors());
            echo json_encode($error);
        } else {
            $data = array('upload_data' => $this->upload->data());
            echo json_encode($data);
        }
    }
}

<form id="upload_form" enctype="multipart/form-data">
    <input type="file" name="userfile" size="20" />
    <input type="submit" value="Upload" />
</form>

upload_max_filesize = 2M
post_max_size = 8M