image_file_whitelist.rb

WHITELISTED_FORMATS = [
  { name: 'png', first_bytes: '89 50 4E 47 0D 0A 1A 0A' },
  { name: 'jpg1', first_bytes: 'FF D8 FF DB' },
  { name: 'jpg2', first_bytes: 'FF D8 FF E0' },
  { name: 'jpg3', first_bytes: 'FF D8 FF E1' },
  { name: 'ico', first_bytes: '00 00 01 00' },
  { name: 'gif1', first_bytes: '47 49 46 38 37 61' },
  { name: 'gif2', first_bytes: '47 49 46 38 39 61' },
  { name: 'tiff1', first_bytes: '49 49 2A 00' },
  { name: 'tiff2', first_bytes: '4D 4D 00 2A' },
  { name: 'pdf', first_bytes: '25 50 44 46' },
  { name: 'bmp', first_bytes: '42 4D' }
]

begin
  file = File.open("/Users/jpb/Desktop/out.png", 'rb')

  found = false
  WHITELISTED_FORMATS.each do |format|
    bytes_to_check = format[:first_bytes].delete(' ')
    bytes_read = file.read(bytes_to_check.length / 2).unpack("H*").first.upcase
    found = bytes_read == bytes_to_check
    break if found
    file.rewind
  end

  if found
    puts 'File is an image'
  else
    puts 'File is not an image'
  end
ensure
  file.close if file
end