public function create($file, $purpose, array $headers = [])
    {
        $response = $this->getClient()->request('POST', 'v1/files', [
            'headers'   => $headers,
            'multipart' => [
                [ 'name' => 'purpose', 'contents' => $purpose ],
                [ 'name' => 'file', 'contents' => fopen($file, 'r') ]
            ],
        ]);

        return json_decode($response->getBody(), true);
    }