How to Modify File Attachment Links to add target="_blank"

This worked for me:
Add the following code into the template.php file of your theme. This is based on the code from bottdev in the link you provided, but updated for drupal 6. The key in the update is that passing attributes to the link function has been modified. See api for function l().

/**
 *  Override for attachments to be target="_blank"
 */
function phptemplate_upload_attachments($files) {
  $header = array(t('Attachment'), t('Size'));
  $rows = array();
  foreach ($files as $file) {
    $file = (object)$file;
    if ($file->list && empty($file->remove)) {
      $href = file_create_url($file->filepath);
      $text = $file->description ? $file->description : $file->filename;
			
      $rows[] = array(l($text, $href, array('attributes' => array('target' => '_blank'))), format_size($file->filesize));
    }
  }
  if (count($rows)) {
    return theme('table', $header, $rows, array('id' => 'attachments'));
  }
}

Open File in New Window (target='_blank')