Add Custom Mime Types - Wordpress

add_filter( 'mime_types', 'wpdocs_add_webp' );

    function wpdocs_add_webp( $wp_get_mime_types ) {
        $wp_get_mime_types['step'] = 'application/octet-stream';
        $wp_get_mime_types['STEP'] = 'application/octet-stream';
        $wp_get_mime_types['x_t'] = 'application/octet-stream';
        return $wp_get_mime_types;
    }