function calculateDataLengthByExtension (torrent, extensions) {
  const files = filterOnExtension(torrent, extensions)
  if (files.length === 0) return 0
  return files
    .map(file => file.length)
    .reduce((a, b) => a + b)
}
Calculate the total data size of file matching one of the provided extensions
@param torrent
@param extensions List of extension to match
@returns {number} total size, of matches found (>= 0)