This one makes an AJAX call directly on the URL for which the assets are retrieved, then an extra WordPress AJAX call to process the list. Sometimes, due to some external factors (e.g. mod_security module from Apache, security plugin or the fact that non-http is forced for the front-end view and the AJAX request will be blocked), this might not work and another choice method might work better. This used to be the only option available, prior to version 1.2.4.4 and is set as default.