Hi, this would need to be 
case '3.0.1': 
  // new checkbox in settings
  $this->setVar('show_attachfile', 0);
case 3.0.2:
  // future upgrades

to have a new 3.0,2 version. Version.php then also needs a 3.0.2 version. In the install part of Installer also the modvar needs to be added, so that for new intalls you also get the new modvar.