function getLargestFileByExtension (torrent, extensions) {
  const files = filterOnExtension(torrent, extensions)
  if (files.length === 0) return undefined
  return files.reduce((a, b) => a.length > b.length ? a : b)
}
Get the largest file of a given torrent, filtered by provided extension
@param torrent Torrent to search in
@param extensions Extension whitelist filter
@returns Torrent file object