public function attachments()
    {
        $files = func_get_args();

        foreach ($files as $file) {
            if (is_string($file)) {
                $this->_attachments[] = ['file' => $file];
            } elseif (is_array($file)) {
                foreach ($file as $key => $value) {
                    $this->_attachments[] = ['name' => $key, 'file' => $value];
                }
            }
        }

        return $this;
    }