From various other sites, and from experiments (thanks Eric Smith) it looks like the associations reported by 'assoc' and 'ftype' aren't necessarily the associations that are actually being used.

Sworddragon:  can you get any useful information out of the Windows registry (e.g., using regedit, searching for Python.File, and looking under shell\open\command), or by setting file associations through the Control Panel?

I don't know enough about Python on Windows to know whether there's any hope that this is a problem that can be solved at the Python end, but I'd guess not.