Validate that filename corresponds to images for the MNIST dataset.

def check_image_file_header(filename):
  """Validate that filename corresponds to images for the MNIST dataset."""
  with tf.gfile.Open(filename, 'rb') as f:
    magic = read32(f)
    read32(f)  # num_images, unused
    rows = read32(f)
    cols = read32(f)
    if magic != 2051:
      raise ValueError('Invalid magic number %d in MNIST file %s' % (magic,
                                                                     f.name))
    if rows != 28 or cols != 28:
      raise ValueError(
          'Invalid MNIST file %s: Expected 28x28 images, found %dx%d' %
          (f.name, rows, cols))