Adding ability to upload photo in more formats

[danielpietzsch]

Adding .apng, .avif and .webp.

Left out .gif because of the potential large file sizes (although .(a)png has a similar problem).

[Schepp]

+1

[danielpietzsch]

I specifically wanted the ability to upload animated images, too, so that I can add photos in the "everyone-is-waving"-style that I've seen from various IWCs. However, it turns out, whatever is exactly processing the images doesn't seem to support animated images. I was able to upload a (rather huge) PNG, but this was turned into an ordinary, non-animated PNG. See here: https://events.indieweb.org/2025/07/homebrew-website-club-düsseldorf-germany-Swt6syowQnsx

The animated image you do see on the page, was only possible by adding the appropriate <img> tag to the "description" of the event, hotlinking an animated AVIF hosted on my own site.

But regardless, I still think it's worth supporting (the non-animated variants of) more image formats, so I would still like this PR to be considered.