+ * AJAX handler for adding custom background context to an attachment.
+ *
+ * Triggered when the user adds a new background image from the
+ * Media Manager.
+ *
+ * @since 4.1.0
+ */
+ 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();
+ }
+
+ /**