testing_magic_bytes.rb

VALID_IMAGE_SIGNATURES = [
  "\x89PNG\r\n\x1A\n".force_encoding(Encoding::ASCII_8BIT), # PNG
  "GIF87a".force_encoding(Encoding::ASCII_8BIT), # GIF87
  "GIF89a".force_encoding(Encoding::ASCII_8BIT), # GIF89
  "\xFF\xD8".force_encoding(Encoding::ASCII_8BIT) # JPEG
].freeze

def file_is_image(temporary_file_path)
  return false unless temporary_file_path

  file_stream = File.new(temporary_file_path, 'r')
  first_eight_bytes = file_stream.readpartial(8)
  file_stream.close

  VALID_IMAGE_SIGNATURES.each do |signature|
    return true if first_eight_bytes.start_with?(signature)
  end

  false
end