public function addFile($file, $options = [])
    {
        if (empty($file) || !is_string($file)) {
            throw new InvalidParamException('Filename must be a string and cannot be empty');
        } elseif (!is_array($options)) {
            throw new InvalidParamException('Options value must be an array');
        }

        $this->files[$file] = $options;

        return $this;
    }