Extract binary magic number (first 4 bytes) - https://en.wikipedia.org/wiki/List_of_file_signatures

magic_number.ex

defmodule MagicNumber do
  def extract_magic_number(<<byte :: size(32), _rest :: binary>>) do
    Integer.to_string(byte, 16)
    |> String.split(~r/.{2}/, include_captures: true, trim: true)
    |> Enum.map(fn n -> "0x#{n}" end)
  end
end

# Usage:
content = File.read!(path)
MagicNumber.extract_magic_number(content)

# Output (png file):
["0x89", "0x50", "0x4E", "0x47"]