`pandoc.image.size` fails on AVIF images

[rnwst]

Running pandoc.image.size on an AVIF image results in the error could not determine image type.

local _, file = pandoc.mediabag.fetch('blue-tit.avif')
local img_data = pandoc.image.size(file)

Unfortunately, GitHub won't let me upload an AVIF file but you can use this one for example.

Pandoc version?
pandoc 3.7.0.2
Features: +server +lua
Scripting engine: Lua 5.4
OS: Linux

[SeanESCA]

Hi, going off your image, https://stackoverflow.com/questions/68314227/what-is-an-avif-file-signature, and ImageSize.hs,
I think we would need to look for 6674 7970 6176 6966 within the first 16 bytes to identify that the image is an AVIF, then add the case for pandoc.image.size to handle AVIF. I haven't come across AVIF before, so I don't know if all AVIF images have metadata to store the dimensions, if the dimensions are stored elsewhere, or if this magic number is even consistent.