Adding attachments to an non-default email template

I would like to be able to add attachments to one of my email templates.

I've found these instructions in /lib/mail_functions.php. But this file only contains the code for the default email templates. I would like to add attachments to a template that I have added. Where can I find these?

Thanks

  // Full Featured Example:

  $errors = sendMessage(array(
    'from'    => "from@example.com",
    'to'      => "to@example.com",
    'subject' => "Enter subject here, supports utf-8 content",
    'text'    => "Text message content",
    'html'    => "<b>HTML</b> message content",

   'headers' => array(
      "CC"              => "cc@example.com",
      "BCC"             => "bcc@example.com",
      "Reply-To"        => "rt@example.com",
      "Return-Path"     => "rp@example.com",
      "x-custom-header" => "12345",
    ),

    'attachments' => array(
      'simple.txt'  => 'A simple text file',
      'dynamic.csv' => $csvData,
      'archive.zip' => $binaryData,
      'image.jpg'   => file_get_contents($imagePath),
    ),