Answer by Sjeiti for How can I enable uploading excel files?

The upload_mimes hook is correct, the mime-type is not (anymore?).

The following is working for WP 5.8.2

function my_upload_mimes( $mimes ) {        return array_merge($mimes, array ('xls' => 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet','xlsx' => 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet','csv' => 'text/csv'        ));}add_filter( 'upload_mimes', 'my_upload_mimes' );