Assuming the type ":touch", Collector.new was calling send(:touch), which instead of triggering method_missing and generating a new collector method, actually invoked the private method `touch` inherited from Object. By generating the method for each mime type as it is registered, the private methods on Object can never be reached by `send`, because the `Collector` will have them before `send` is called on it. To do this, a callback mechanism was added to Mime::Type This allows someone to add a callback for whenever a new mime type is registered. The callback then gets called with the new mime as a parameter. This is then used in AbstractController::Collector to generate new collector methods after each mime is registered.