Bueno lo probe pero igual ala hora de adjuntar ignora el salto de linea del txt generado
require 'lib/geekMail-1.0.php';
$geekMail = new geekMail();
$geekMail->setMailType('html');
$geekMail->from('
[email protected]', 'prueba');
$geekMail->to('
[email protected]');
$geekMail->subject('Reporte');
$geekMail->message('Enviando');
$geekMail->attach(nl2br('data.txt'));
if (!$geekMail->send())
{
$errors = $geekMail->getDebugger();
print_r($errors);
}