valid_image.rb

def valid_image?(filename)

  File.open(filename, "rb") do |file|
    begin
      header = file.read(8)
      file.seek(-12, IO::SEEK_END)
      footer = file.read(12)
    rescue
      return false
    end

    # jpg
    return true if filename.end_with?('.jpg') and
                   header[0, 2].unpack("H*") == ["ffd8"] and
                   footer[-2, 2].unpack("H*") == ["ffd9"]

    # gif
    return true if filename.end_with?('.gif') and
                   header[0, 3].unpack("A*") == ["GIF"] and
                   footer[-1, 1].unpack("H*") == ["3b"]

    # png
    return true if filename.end_with?('.png') and
                   header[0, 8].unpack("H*") == ["89504e470d0a1a0a"] and
                   footer[-12, 12].unpack("H*") == ["0000000049454e44ae426082"]
  end

  false

end