Programmatically set class on extended field type
This is untested, but I'm pretty sure you just need to change the list class for the field types using hook_field_info_alter():

function ablecore_field_info_alter(&amp;$info) {
  foreach ($info as &amp;$field) {
    if ($field['list_class'] == '\Drupal\file\Plugin\Field\FieldType\FileFieldItemList') {
      $field['list_class'] = '\Drupal\ablecore\Plugin\Field\FieldType\AbleFileFieldItemList';
    }
  }
}