private function prepareAttachments($attachments = null)
    {
        $file_params = [];

        if ($attachments) {
            $counter = 1;

            foreach ($attachments as $attachment) {
                $path = is_array($attachment) ? $attachment[0] : $attachment;

                if (is_readable($path)) {
                    $file_params['attachment_'.$counter++] = $attachment;
                } else {
                    throw new FileNotReadable($attachment);
                }
            }
        }

        return $file_params;
    }