wp_ajax_save_attachment_order()

function wp_ajax_save_attachment_order() {
	if ( ! isset( $_REQUEST['post_id'] ) )
		wp_send_json_error();

	if ( ! $post_id = absint( $_REQUEST['post_id'] ) )
		wp_send_json_error();

	if ( empty( $_REQUEST['attachments'] ) )
		wp_send_json_error();

	check_ajax_referer( 'update-post_' . $post_id, 'nonce' );

	$attachments = $_REQUEST['attachments'];

	if ( ! current_user_can( 'edit_post', $post_id ) )
		wp_send_json_error();

	foreach ( $attachments as $attachment_id => $menu_order ) {
		if ( ! current_user_can( 'edit_post', $attachment_id ) )
			continue;
		if ( ! $attachment = get_post( $attachment_id ) )
			continue;
		if ( 'attachment' != $attachment->post_type )
			continue;

		wp_update_post( array( 'ID' => $attachment_id, 'menu_order' => $menu_order ) );
	}

	wp_send_json_success();
}