Add avif MIME type

[xt0rted]

Add avif MIME type

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Summary of the changes (Less than 80 chars)

Add avif extension to list of files used by static file middleware.

Description

This is a common image format supported by all major browsers now¹, adding it will make it easier for people to use this format.

There's a couple Http.config files used in unit tests where this wasn't added. I didn't bother since webp wasn't in there, but I can add them if needed.

Fixes #39984

Footnotes

1. https://caniuse.com/avif ↩

[javiercn]

@xt0rted thanks for the contribution!