GLTFLoader: Preserve unknown glTF extensions on "mesh" properties (#25948)

examples/jsm/loaders/GLTFLoader.js

@@ -3648,12 +3648,16 @@ class GLTFParser {
 
 			if ( meshes.length === 1 ) {
 
+				if ( meshDef.extensions ) addUnknownExtensionsToUserData( extensions, meshes[ 0 ], meshDef );
+
 				return meshes[ 0 ];
 
 			}
 
 			const group = new Group();
 
+			if ( meshDef.extensions ) addUnknownExtensionsToUserData( extensions, group, meshDef );
+
 			parser.associations.set( group, { meshes: meshIndex } );
 
 			for ( let i = 0, il = meshes.length; i < il; i ++ ) {