class PlgSystemWf_filesystem_events extends JPlugin
{
    /**
     * onWfFileSystemGetRootDir
     * @param   string  $path  The relative root directory path, eg: "images".
     * @return  void
     */
    public function onWfFileSystemGetRootDir(&$path)
    {
        $app = JFactory::getApplication();
        $user = JFactory::getUser();

        // into images field file browser...
        if ($app->input->getCmd('fieldid') === 'jform_images_image_intro') {
            // append the username to the current root file directory path
            $path .= '/' . $user->username;
        }
    }
}