public function addAttachment($filename, $contents)
    {
        $this->connection()->upload($this->endpoint . '/' . $this->id . '/attachments', [
            'multipart' => [
                [
                    'name' => 'file',
                    'contents' => $contents,
                    'filename' => $filename,
                ],
            ],
        ]);

        return $this;
    }