WebP response failed mime-type test

That means the rewrite rules were added to your site, but they don’t work. Most often, this is due to a caching/proxy system in front of your site that doesn’t “vary” the cache based on the “Accept” HTTP header. Cloudflare is a very common one (though it’s usually auto-detected).
In such cases, to successfully deliver WebP images to browsers that support it, and the original image to those that do not, I recommend you use the JS WebP delivery method instead (unless you can have the cache/proxy configured correctly).

Thanks, it says it failed but for some reason, google page speed is detecting the images, could it be some other plugin had made the changes before?

No, it’s not from some other plugin. If the self-test failed, that means the proxy/cache system is storing whichever copies of the images it gets first.
If those are WebP (and the odds favor that scenario), then non-WebP browsers will have broken images. If it were a non-WebP browser that visited a page first, then no WebP images would be delivered until the cache expires–and then whatever browser visits first, those images are stuck in the cache and will not be varied.

In your case, the cache has WebP images stuck in cache, and Google correctly sees those. But anyone who visits your site with an older browser (typically 1 in 20 visitors) will get broken images.

Thanks, ill try the JS WebP delivery method.

I’ve just gone back to the plugin and I get this message today.

WebP Rules verified successfully, should I still make the changes ?

Nope no change should be needed, but that IS a bit strange. I’m trying to think if the test image was already cached, what that would do. So if it was cached as PNG (from being displayed on the settings prior to the rewrite rule insertion), that would cause the self-test to get a PNG when it was expecting WebP. Looking at the code, that is indeed the case that triggers that specific error–getting a PNG response when the Accept header is set to image/webp.
I’ll have to keep that in mind if/when we see this come up again!

Thanks for the quick replies and great plugin.