No support for .tar.xz

[steve-thousand]

It looks like this project may not be active anymore, but I am checking to see if anyone knows why there is not support for .tar.xz in the FileTypes map https://github.com/thrau/jarchivelib/blob/master/src/main/java/org/rauschig/jarchivelib/FileType.java#L47-L51

If it were not package-private I would try passing my own new FileType(org.rauschig.jarchivelib.ArchiveFormat.TAR, org.rauschig.jarchivelib.CompressionType.XZ) filetype to the ArchiverFactory

[anm-cb]

#74