def save(dst_file_path = nil)
      dst_file_path ||= root.source_file_path

      extension = File.extname(dst_file_path)
      unless %w{.xlsx .xlsm}.include?(extension.downcase)
        raise "Unsupported extension: #{extension} (only .xlsx and .xlsm files are supported)."
      end

      File.open(dst_file_path, "wb") { |output_file| FileUtils.copy_stream(root.stream, output_file) }

      return dst_file_path
    end
def save(dst_file_path = nil)
      dst_file_path ||= root.source_file_path

      extension = File.extname(dst_file_path)
      unless %w{.xlsx .xlsm}.include?(extension.downcase)
        raise "Unsupported extension: #{extension} (only .xlsx and .xlsm files are supported)."
      end

      File.open(dst_file_path, "wb") { |output_file| FileUtils.copy_stream(root.stream, output_file) }

      return dst_file_path
    end
Save the resulting XLSX file to the specified location