Please add APNG filetype

[bencresty]

Animated GIF is old fashioned. APNG is the modern replacement, so Animated PNG.
We can add this filetype by hand, but it's such an important file format for the web that IMO it should be in the default filetypes allowed by Craft. Especially because it can not be a security issue (it doesn't execute code).

APNG is supported for quite some time now in all modern browsers:
https://caniuse.com/apng

Not sure if adding this extension to the Image file kind and it's definitely not considered video, so I propose a new type, called 'Animated Image'. This could perhaps even contain animated gifs.

BTW Animated PNGs (APNG) can also have the PNG extension. But when we upload it like this in Craft it's converted to a 'frozen' PNG. So it's not possible to use PNG for APNG's now.

Mime type is image/apng
(https://wiki.mozilla.org/APNG_Specification)

[Edit] I see we cannot 'just' add the APNG file type ourselves as we are missing the needed properties like width and height of the image