On J2.5, try it this way:

if (
  $option == 'com_k2'
  and $view == 'item'
) {
  $newid = (int)$id;
  $query = 'select itemID from #';
  $query .= '__k2_attachments where itemId = ' . $newid;
  $db->setQuery($query);
  $result = $db->loadResult();
  if ($result) return XXX;
  // replace XXX with the module ID of the K2 content module.
  // the target module will only show if you are on a K2 item view
  // page for an item with an attachment on it.
}