When a parameter is a file, for ex., in "For every matching file, invoke one build", those files are being saved into /var/lib/jenkins/jobs/$job/builds/$buildid/$varname/$file_pattern location. I haven't found any way to delete that stuff from that location – "Delete workspace" will act on /var/lib/jenkins/workspace/ and "Discard old builds" -> "artifacts cleanup" checkbox does nothing.

Thanks.