Adds apng sniffing when generating thumbnails with retained ratios to
serve the original image files, as we do for GIF images, to prevent
the image being resized to a static version.
Is more tricky than GIF since apng file mimes and extensions
are the same as png, we have to detect part of the file header
to sniff the type. Means we have to sniff at a later stage
than GIF since we have to load the image file data.
Made some changes to the image thubmnail caching while doing
this work to fit in with this handling.
Added test to cover.
For #3136.
