The fileupload plugin is not using the xwiki temporary directory when xwiki.upload.tempdir is not set

Right now if xwiki.upload.tempdir is not set, then commons fileupload will default to using the java.io.tmpdir system property (which doesn't correspond to xwiki's temporary directory - which uses the servlet tmp dir).

See https://forum.xwiki.org/t/error-when-importing-backup/6574/13 for some context of how this issue was found.