How to enable thumbnails for ?.webp? format

You don’t, yet. But you can help on the tickets.
In the #core dev chat yesterday, Mike Schroder said:

I think things are getting pretty close for an initial commit for WebP support, so any testing and feedback on it, and the related filter for output, would be appreciated:
#35725 and the filter in #52867

This plugin generates WebP versions of all images in the media library (all sizes), while keeping the original images as a fallback: https://www.ads-software.com/plugins/webp-converter-for-media/

@joyously But we all know this format is much smaller and more convenient. So the WordPress development team should add this format to WordPress…

@pidengmor I tested this plugin. It was not appropriate

I’m looking for a suitable solution to automatically generate all smaller sizes after uploading a ?.webp? image

With that plugin you can upload stardard format images (e.g. jpeg/png). Let WordPress gerenate all the different sizes, and then use the plugin to create webp versions of those image sizes.

I want to upload webp images directly

Then you will need to get involved in testing the filter linked to by Joy above.

Very glad to see WordPress is adding native support for WebP!

Google has been recommending this for some time:

https://web.dev/serve-images-webp/

I too want to upload .webp files directly, not have them converted as second-generation from other already-compressed images like jpeg…

There’s no reason not to natively support .webp like any other uploaded image format (at perhaps half the file size), now that all major browsers support it:

https://caniuse.com/webp

Thanks