Thanks, your suggestion was perfect.
defp download(1, id, file_name, size), do: Downloader.download(Downloader1, id, file_name, size)
defp download(2, id, file_name, size), do: Downloader.download(Downloader2, id, file_name, size)
defp download(3, id, file_name, size), do: Downloader.download(Downloader3, id, file_name, size)
defp download(4, id, file_name, size), do: Downloader.download(Downloader4, id, file_name, size)
defp download(5, id, file_name, size), do: Downloader.download(Downloader5, id, file_name, size)