What?

We could simplify the code by entirely removing the WP_Font_Family_Utils::has_font_mime_type() function and its tests.
This was a workaround when we didn't have the wp_handle_upload() mime type checking in place.

Why?

We can remove it now because it's no longer necessary. The mime type check of the downloaded/uploaded files is done by the core function wp_handle_upload, and a custom check is no longer required.