'True if this image is in PNG format.'

@property
def is_png(self):

    if hasattr(self, '_m_is_png'):
        return (self._m_is_png if hasattr(self, '_m_is_png') else None)
    self._m_is_png = (self.png_header == struct.pack('8b', (-119), 80, 78, 71, 13, 10, 26, 10))
    return (self._m_is_png if hasattr(self, '_m_is_png') else None)
