In win10 Explorer, I've set "open with" of .webp files to open with irfanview, and it works, opening in irfan, with the icon itself also showing as irfan's. In Dopus 12.17, the icon shows as irfan's, but double-clicking opens it in dopus's d8viewer. I checked in "Settings | File types" in dopus, and it shows webp as opening with irfan... (I don't see this addressed in subsequent dopus changelogs, but can update dopus if that will help). (I'm making this change of openwith because the dopus viewer doesn't yet handle animated .webp files; I think the time has come to add that ability).
You could disable the webp plugin (Preferences / Viewer / Plugins) and that should be enough if it's only webp that you don't want to open in the Opus viewer.
Quick response! While that may work, I’m moreso curious about why dopus isn’t following the registry setting. Double-clicking on a webp file should open the file into whatever has been set in the registry (or, if dopus keeps its own list, whatever’s in its file type settings), right?
If you have Preferences / File Operations / Double-click on Files / Use internal picture viewer turned on then Opus will use its viewer to show any files it recognises as images.