[Joker_vD]

> if the url with the extension gets past around instead of the original (as would inevitably be done) you're back to square one.

Why? It has no "Vary" header, and it's the one that's supposed to get cached anyhow.

[bmandale]

If people see it in the url bar and copy paste it from there. In the case of images if they "copy image url".