"open" does not work (for xz file)
I needed to extract a .tar.xz file, so I started RAR Extractor Star and pulled down "FIle" -> "Open...". I browsed to the right directory and saw the .tar.xz file, but it was grey and could not be selected.
Fortunately, I was able to go into Finder and double-click the .tar.xz file. That brought up RAR Extractor Star, and I was able to extract.
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RAR Extractor - Unarchiver Pro