https://github.com/KhronosGroup/glTF/blob/main/specification/2.0/schema/glTFProperty.schema.json https://github.com/KhronosGroup/glTF/blob/main/specification/2.0/schema/extension.schema.json https://github.com/KhronosGroup/glTF/blob/main/specification/2.0/schema/extras.schema.json
JSON object with extension-specific objects.
Although extras MAY have any type, it is common for applications to store and access custom data as key/value pairs. Therefore, extras SHOULD be a JSON object rather than a primitive value for best portability.
extras