public function uploadRichMenuImage($richMenuId, $imagePath, $contentType)
    {
        $url = sprintf('%s/v2/bot/richmenu/%s/content', $this->endpointBase, urlencode($richMenuId));
        return $this->httpClient->post(
            $url,
            [
                '__file' => $imagePath,
                '__type' => $contentType,
            ],
            [ "Content-Type: $contentType" ]
        );
    }