Hi @webulous!
I was testing with Edge 111.0.1661.41 in internet explorer mode and devtools are not available there so I coudn’t check the devtools network tab.
Today I tested with Safari on an iPad mini 2 running iOS 12.5.7. Using Safaris web inspector on a Mac I saw that the content-type in response headers was indeed set to image/jpeg even though the image URL in source references the WebP image, so it should work. But the images are not showing in content and preview tab in web inspector says “Resource has no content”.
On supported devices the WebP images work as expected.