I pinged Firaxis about this, and got this response:
Mike B. FIRAXIS wrote on Nov 24, 2003 09:14 AM:
I asked Jesse Smith (the producer) about this and he told me that Atari will be making the Extras available for download. He also told me that it is ok for people to post the contents of the Extras folder (winter terrain, european terrain, ww2 units, medieval japan units, prehistoric units).