[D7] Allow image fields to use any extensions the current image toolkit supports (instead of hard-coding jpg, png and gif only)

I don't see this getting into D7, can we get it in d10 or 11?

AVIF & WebP look ready to roll though:
https://caniuse.com/?search=avif
https://caniuse.com/?search=webp

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@mgifford this is a D7 backport issue. The original issue is here: 🐛 Allow image fields to use any extensions the current image toolkit supports (instead of hard-coding jpg, png and gif only) Fixed and was fixed 6 years ago, so it should be in D10+ already.

Since the last patch no longer applied to the 7.x branch, I created a MR.