identify_image_format.rb

  IMAGE_MAGIC = {
    'JPG' => 'ffd8',
    'BMP' => '424d',
    'TIFF-LE' => '49492a00',
    'TIFF-BE' => '4d4d002a',
    'GIF87a' => '474946383761',
    'GIF89a' => '474946383961',
    'PNG' => '89504e470d0a1a0a',
  }.freeze
  IMAGE_MAGIC_LEN = (IMAGE_MAGIC.values.map(&:size).max / 2.0).ceil

  def identify_image_format(f)
    hex = f.read(IMAGE_MAGIC_LEN).unpack('H*').first
    f.rewind
    IMAGE_MAGIC.each {|format, magic| return format if hex.start_with?(magic) }
    return false
  end