What I don't understand is the way the archive is "saved":
Using bz2 decompression downloading file:///tftpboot/eltf/standard/Enea_Linux_7.0_ raspberrypi3-64_enea-image-standard_2017-12-14_16-47-18/ images/raspberrypi3-64/enea-image-standard-raspberrypi3-64.tar.bz2 saving as /var/lib/lava/dispatcher/tmp/525/tftp-deploy-jBJXcy/ nfsrootfs/enea-image-standard-raspberrypi3-64.tar
Just to answser this question: the archive is decompressed on the fly (while downloading) so the resulting file is a ".tar".