tw_ee_filter_RegistrationsReport__get_filename.php

<?php //Do not include the opening PHP tag if you already have one.

add_filter('FHEE__EventEspressoBatchRequest__JobHandlers__RegistrationsReport__get_filename', 'tw_ee_filter_RegistrationsReport__get_filename', 10);
function tw_ee_filter_RegistrationsReport__get_filename( $filename ) {
    if( !empty($_REQUEST['EVT_ID'])) {
        $event = EEM_Event::instance()->get_one_by_ID( absint($_REQUEST['EVT_ID']) );
        if( $event instanceof EE_Event ) {
            return sprintf(
            "event-espresso-registrations-{$event->slug()}-%s.csv",
            str_replace(array(':', ' '), '-', current_time('mysql'))
            );
        }
    }
    return $filename;
}