Avif support

[fatk]

Hi there,

I've been using timmy for years and it's been awesome, so thank you!

Recently, Wordpress (as of 6.5) and Timber as of (2.3) have enabled support for avif format.
I've successfully enabled avif-only image generation by using the image_editor_output_format wordpress filter, as well as timber/image/new_url and timber/image/new_path filters.

However, Timmy's ignore_attachment method has a hardcoded allowed file extension array which prevents processing avif files.
Do you think you could either add avif (and webp) to the array, or better, create a new filter for users to be able to modify supported extensions?

Thanks again!