As of Tiled 1.11, extensions can be written in plain JavaScript files or as JavaScript modules. However, Tiled actually uses a QQmlEngine internally, which is able to load QML files as well.

See Next generation Tiled extensions for more information about this idea.

When the necessary libraries and plugins are available, Qt.createComponent can already be used to instantiate QML types. A nicer integration should be possible when Tiled recognizes QML files and creates QQmlComponent instances with those files and creates objects from them.