RealPlayer has probably taken over the registry settings.
Reinstalling FormatPlayer might make it take the over again.
(I'm assuming you don't see either of them in File Explorer's viewer pane, but if you do there may be another way to adjust which is used. Let me know.)
Alternatively, enable the Opus movie plugin and (for Opus 12) install LAV Filters (see here), which should let you view videos via that, without the registry mattering much.