'Pre-reads first 8 bytes of the image to determine if it\'s an
embedded PNG file.'

@property
def png_header(self):

    if hasattr(self, '_m_png_header'):
        return (self._m_png_header if hasattr(self, '_m_png_header') else None)
    _pos = self._io.pos()
    self._io.seek(self.ofs_img)
    self._m_png_header = self._io.read_bytes(8)
    self._io.seek(_pos)
    return (self._m_png_header if hasattr(self, '_m_png_header') else None)
