This diff enables read-only support for XAR achives in libarchiveplugin. It seems libarchive has partial read-write support for this format, but I encountered an error when attempting to create a Xar archive.
Things to consider:
Shared-mime-info 1.6 (released in February 2016) contains the mimetype for XAR. Should we install the mimetype for XAR ourselves in case the user has an older shared-mime-info?