Hi,
Actually, this isn’t an ACF issue. You need to add this file type to the list of WordPress mime types which are allowed for upload. You can find the default allowed types here : https://codex.wordpress.org/Function_Reference/get_allowed_mime_types
To add your own to the list, you can take a look at this tutorial : https://www.wpbeginner.com/wp-tutorials/how-to-add-additional-file-types-to-be-uploaded-in-wordpress/
