mime-types: add image/avif

[ulidtko]

Greetings! Quick addition here, .avif → image/avif.

AVIF is a new-ish open standard for images, based on the AV1 codec.

It is registered in IANA: https://www.iana.org/assignments/media-types/media-types.xhtml#image

Checklist:

• Bumped the version number
• Update the Changelog.md file with a link to your PR
• Check that CI passes (or if it fails, for reasons unrelated to your change, like CI timeouts)