__init__.py

  if size < MIN_PNG_SIZE:
      raise errors.ClientException('`image` is not a valid image')
  if size > MAX_IMAGE_SIZE:
      raise errors.ClientException('`image` is too big. Max: {0} '
                                   'bytes'.format(MAX_IMAGE_SIZE))
  first_bytes = image.read(MIN_PNG_SIZE)
  image.seek(0)
  if first_bytes.startswith(PNG_HEADER):
      image_type = 'png'
  elif first_bytes.startswith(JPEG_HEADER):
      if size < MIN_JPEG_SIZE:
          raise errors.ClientException('`image` is not a valid image')
      image_type = 'jpg'
  else:
      raise errors.ClientException('`image` must be either jpg or '
                                   'png.')