Fileinfo Extension is Required

[sbossarte]

The description for the fileinfo extension indicates that it is recommended to have, but not required. However, uploading images in Craft requires the fileinfo extension in order to validate Mime types.

https://github.com/craftcms/cms/blob/ced62c5a768973318c53cada422d0ff911cfe73e/src/image/Raster.php#L166

Considering the fileinfo extension is enabled by default in any version of PHP that Craft 3 supports, it's probably unlikely that it would be disabled in any sane environment, but just for the sake of correctness it might be good to update its description and ensure the issue is caught as part of the pre-install check.