Get missing file type from magic numbers in Elixir

get_file_type.ex

defmodule Mix.Tasks.GetFileType do
  use Mix.Task

  def run([filename]) do
    File.read!(filename)
    |> check_magic_bytes()
    |> IO.puts()
  end

  # Here's some basic file signatures, but you can easily add others as needed.
  # https://en.wikipedia.org/wiki/List_of_file_signatures
  defp check_magic_bytes(<<137, 80, 78, 71, 13, 10, 26, 10, _::binary>>), do: "png"
  defp check_magic_bytes(<<37, 80, 68, 70, 45, _::binary>>), do: "pdf"
  defp check_magic_bytes(<<73, 73, 42, 0, _::binary>>), do: "tiff"
  defp check_magic_bytes(<<77, 77, 0, 42, _::binary>>), do: "tiff"
  defp check_magic_bytes(<<255, 216, 255, _::binary>>), do: "jpg"
  defp check_magic_bytes(_), do: "unknown"
end

# Usage:
# $ mix get_file_type ./some_png_without_extension
# png
# $ mix get_file_type ./something_not_on_list
# unknown