Update index.js

I tested it on my local machine. Now the Envelopment map is downloaded correctly

Playground/index.js

@@ -299,7 +299,7 @@
             if (textures[index].isCube) {
                 if (textures[index]._extensions) {
                     for (var i = 0; i < 6; i++) {
-                        textures.push({ name: textures[index].name + textures[index]._extensions });
+                        textures.push({ name: textures[index].name + textures[index]._extensions[i] });
                     }
                 }
                 else {
@@ -596,4 +596,4 @@
         }
     };
     xhr.send(null);
-})();
+})();