D6 prohibit upload file extensions
Not directly, but you could do that in your own custom form validation.  For example:

function YOURMODULE_form_alter(&amp;$form, $form_state, $form_id) {

  switch ($form_id) {

    case 'YOUR_CONTENT_TYPE_node_form':

      $form['#validate'][]='YOURMODULE_valid_extensions';
      break;

    // any other forms here

  }

  // any other form alters here

}

function YOURMODULE_valid_extensions($form, &amp;$form_state) {

  // $whitelist=array('txt', 'doc', 'pdf'); 
  $blacklist=array('xxx', 'exe', 'php');

  /* I am assuming that this is a single value field ([0] below) */
  $extension=pathinfo($form_state['values']['field_YOURFIELD'][0]['filename'], PATHINFO_EXTENSION);

  // if (in_array($extension, $whitelist)) {
  //   return;
  // }

  if (in_array($extension, $blacklist)) {
    form_set_error('field_YOURFIELD', t('Invalid file extension'));
  }

}


and if you leave the extensions field blank, filefield won't do any validation, but you will in the second function above.  I've also put in //'s basically what filefield is doing if you wanted to incorporate both.  If you do go this route, you will have to put some verbiage in your help section as the widget will most probably say all extensions are valid :)