Add MIME type for AVIF images
AVIF is an image format for still and animated images, with both lossy and lossless compression, derived from the AV1 bitstream format.
The HEIF file format is reused as a container, though the extension is most likely going to be .avif.
I was not able to obtain detailed documentation on the HEIF container format, but at offset 0x0014, I find either heic or avif in the sample images, respectively for heic and avif files. Obviously, a full parser would be needed to completely distinguish between files.
As a side note, it would be nice to also support generic HEIF files.