Added Mockup placement type constant.

Author: briedis

src/Structures/Placements.php

@@ -46,6 +46,11 @@ class Placements
      */
     const TYPE_LABEL_OUTSIDE = 'label_outside';
 
+    /**
+     * Mockup of the product, can be used when submitting orders, etc
+     */
+    const TYPE_MOCKUP = 'mockup';
+
     /**
      * @var array
      */