amp-img: short circuit webp fallback for iOS

[aghassemi]

if src of an amp-img points to a .webp file, we should be able to short circuit to fallback immediately without waiting for img load to fail. This will remove one extra download in iOS case.

There will be cases where file extension is not present which this won't support. Also supporting srcset requires parsing and even in that case a mixed srcset will not work so skipping srcset all together for this.

There is a small chance some files with .webp extension are not actually .webp and may return some other format (maybe depending on requester) and rely on browser content-sniffing to get around the discrepancy. I think this is acceptable but we should put the new behaviour behind an experiment first just in case.

[Gregable]

There is a related old issue for this: #3517

[ampprojectbot]

This issue hasn't been updated in awhile. @aghassemi Do you have any updates?

[aghassemi]

since cache server-side detects support and responds with webp (even for .jpg extension), I don't think this is something we want to do. Documenting the server-side solution (#21912) is the right way to go.