如何使post附件在save_post操作上可用？

function add_attachment_meta_box() {
    add_meta_box(
        'attachment_meta_box',
        '附件',
        'render_attachment_meta_box',
        'post',
        'normal',
        'high'
    );
}

function render_attachment_meta_box($post) {
    wp_nonce_field('attachment_meta_box', 'attachment_meta_box_nonce');
    $attachment_id = get_post_meta($post->ID, 'attachment_id', true);
    // 在这里显示附件信息或上传附件的表单
}
add_action('add_meta_boxes', 'add_attachment_meta_box');

function save_attachment_meta_box($post_id) {
    if (!isset($_POST['attachment_meta_box_nonce']) || !wp_verify_nonce($_POST['attachment_meta_box_nonce'], 'attachment_meta_box')) {
        return;
    }
    if (defined('DOING_AUTOSAVE') && DOING_AUTOSAVE) {
        return;
    }
    if (!current_user_can('edit_post', $post_id)) {
        return;
    }
    if (!isset($_FILES['attachment'])) {
        return;
    }
    $attachment = $_FILES['attachment'];
    $upload_overrides = array('test_form' => false);
    $movefile = wp_handle_upload($attachment, $upload_overrides);
    if ($movefile && !isset($movefile['error'])) {
        $attachment_id = wp_insert_attachment(array(
            'post_mime_type' => $movefile['type'],
            'post_title' => sanitize_file_name($attachment['name']),
            'post_content' => '',
            'post_status' => 'inherit'
        ), $movefile['file'], $post_id);
        if ($attachment_id) {
            update_post_meta($post_id, 'attachment_id', $attachment_id);
        }
    }
}
add_action('save_post', 'save_attachment_meta_box');