@ferrier What the URL says is irrelevant (and may be wrong). For files in web pages the "content-type" in the HTTP response is essential. And of course the actual file data is also important. GIF and WEBP are two totally different file formats and therefore easy detectable for Opera.
As web browser usually don't convert any file formats I assume that if Opera wants to save an image as *.webp it actually is a webp image. You can verify that by downloading the file using a command-line tool like wget and the open the file in an image viewer that displays the actual image type.