Version: (using KDE 4.4.1) Installed from: Slackware Packages ark as of kde 4.4.1 is unable to open xz compressed tarballs (.txz or .tar.xz), as for example used by slackware's packages. In kde 4.3.x I was presented with a dialog where I could choose a compression format. Choosing LZMA there always worked fine. In 4.4.x I get confronted with the "Unrecognized archive" error and no way to force opening it as lzma. I get the same result for archives ending with ".tar.lzma" and ".tlz". If I rename it to "*.tar.lz" I can again choose the compression format and everything works fine again if I choose LZMA.
Suport for txz files is already present - you need to update your libarchive package. For more information, see bug 229036. *** This bug has been marked as a duplicate of bug 229036 ***
This has now been fixed downstream. Thanks for the hint :)