public function thumbnails($file, $format = 'JPEG', $size = 'small')
    {
        // Only allow php response format for this call
        if ($this->responseFormat !== 'php') {
            throw new Exception('This method only supports the `php` response format');
        }
        
        $format = strtoupper($format);
        // If $format is not 'PNG', default to 'JPEG'
        if ($format != 'PNG') $format = 'JPEG';
        
        $size = strtolower($size);
        $sizes = array('s', 'm', 'l', 'xl', 'small', 'medium', 'large');
        // If $size is not valid, default to 'small'
        if (!in_array($size, $sizes)) $size = 'small';
        
        $call = 'thumbnails/' . $this->root . '/' . $this->encodePath($file);
        $params = array('format' => $format, 'size' => $size);
        $response = $this->fetch('GET', self::CONTENT_URL, $call, $params);
        
        return array(
            'name' => basename($file),
            'mime' => $this->getMimeType($response['body']),
            'meta' => json_decode($response['headers']['x-dropbox-metadata']),
            'data' => $response['body'],
        );
    }