Attach generated PDF to user notification Email

Hi,
I am using the gform_pre_submission hook to generate a pdf certificate for my forms which is working great. I'm using gform_pre_submission because I am changing the content of one of the fields to the path of the generated file.

Now I am trying to get this file attached to the user notification email but can't get that to work. I am guessing I have to use the gform_user_notification_attachments hook but don't know how to use it.

Here is my code for the gform_pre_submission function, $_POST["input_20"] contains the path to the file I want to attach:

add_action("gform_pre_submission", "set_post_content", 10, 2);

	function set_post_content($form){
	 if ( $_POST['input_21'] = 'Test' and $_POST['input_2'] > 6 )	{
    $post_url = 'pdf_zertifikat_generator.php';
    $body = array(
        'datum' => $_POST['input_11'],
        'quiz_name' => $_POST['input_6'],
        'cme' => $_POST['input_13'],
        'bp_login' => $_POST['input_19'],
        'wp_email' => $_POST['input_10'],
		'bp_id' => $_POST['input_17'],
		'score' => $_POST['input_2'],
		'VNR' => $_POST['input_22']
        ); 

	if ( $_POST['input_2']>6 )	{
	$request = new WP_Http();
    $response = $request->post($post_url, array('body' => $body));

	$_POST["input_20"] = "../pdf/zertifikate/" . $_POST['input_17'] . "_" . $_POST['input_22'] . ".pdf";
	}
	}
}