This is what it has to be:
{{response.headers['Content-Disposition'] = 'attachment;filename=event_%s_%s.ics' % ((IS_SLUG()(organization.name)[0]), row.id)}}
This is what it has to be:
{{response.headers['Content-Disposition'] = 'attachment;filename=event_%s_%s.ics' % ((IS_SLUG()(organization.name)[0]), row.id)}}