Custom_Background::ajax_background_add

	public function ajax_background_add() {
		check_ajax_referer( 'background-add', 'nonce' );

		if ( ! current_user_can( 'edit_theme_options' ) ) {
			wp_send_json_error();
		}

		$attachment_id = absint( $_POST['attachment_id'] );
		if ( $attachment_id < 1 ) {
			wp_send_json_error();
		}

		update_post_meta( $attachment_id, '_wp_attachment_is_custom_background', get_stylesheet() );

		wp_send_json_success();
	}