There are two ways, but similar result.
In one, you select the file and choose "Open With..." and pick the application you want and check the box "Always open with..." or whatever.
In the other you hunt down the association in the Registry and remove it.
The downside is that double clicking the file won't open Pro/E Creo whatever, based on the extension, but since I never do that, it's not a loss.
PTC stakes out .1 to .400 or so. Correction - from .1 to .999, in WF5.