Adding attachment from a mailer snippet not working

... some processing to setup the email...

// Attach the terms & conditions document to the email.
$tcFile = $modx->config['site_url'] . 'assets/documents/TC.pdf';

$modx->getService('mail', 'mail.modPHPMailer');
$modx->mail->set(modMail::MAIL_BODY, $message);
$modx->mail->set(modMail::MAIL_FROM, $emailAddress);
$modx->mail->set(modMail::MAIL_FROM_NAME, $emailAddress);
$modx->mail->set(modMail::MAIL_SENDER, $emailAddress);
$modx->mail->set(modMail::MAIL_SUBJECT, '...']);
$modx->mail->address('to', $object->getTVValue('email'));
$modx->mail->address('reply-to', $emailAddress);

// Add the attachment
$modx->mail->mailer->AddAttachment($tcFile);

$modx->mail->setHTML(true);
if (!$modx->mail->send()) {
    $modx->log(modX::LOG_LEVEL_ERROR,'An error occurred while trying to send the email: '.$modx->mail->mailer->ErrorInfo);
}
$modx->mail->reset();

$tcFile = $modx->getOption('base_path') . 'assets/documents/TC.pdf';