Yeah, I thought that.
But I also thought that the plugin might work as a hook, i.e. whenever a file is opened in paint.net, it checks if there is a plugin hooked in, which in turn checks if it should handle the opening. In case of the webp plugin I figured that it only looks for the file extension (PNG) and decides hold still and process back to paint.net, which fails opening the file because it is a secret WEBP file.
If that is true, could the plugin not check the file content to see if it is a WEBP file in disguise and open it regardless of the extension, or otherwise pass control back to paint.net? Or will it not be called for PNG files at all?
But I guess you would know that better than me