Has been two days since your reply, so maybe you’ve solved the problem.
Anyway, I have a few remarks and questions.
I have no idea how OSX (Finder) knows which application opens certain files. On installing an application that information must be exchanged somehow.
Yes, it’s still possible, but it’s not recommended to fiddle around with those settings. See part of a MacPilot7 window below. Read the warning.
When you ‘Get Info’ of the (should be) .rw6 file, what information do you see?
Does the extension exist or it is absent?
In the first picture below yo see a ‘Get Info’ window of some .rw6 file.
The second one shows the info after removing the .rw6 extension.
The icon changes to a Folder icon and I can’t open the file by dropping it on the RapidWeaver icon in the Dock like you can.
I don’t know if re-installing RapidWeaver could fix it. You then should probably remove every trace of RW’s existence before re-installing the software. Maybe a more knowledgeable person could give his opinion about that. e.g. @nikf
Sorry, I have no fix, had thought LaunchServices should fix it.