Oops! Now here comes the real answer :-)

On Wed, 10 Nov 1999 Torsten.Bergmann at phaidros.com wrote:

> Squeak does also support .movie files. 
> Any ideas or pointers to example movie files?

It's a series of uncompressed binary Squeak forms. Look at 
MoviePlayerMorph's class comment to see an example, which boils down to
Form>>#writeOnMovie:.

  -Bert-