At some point, the behavior of the Python tarfile library seems to have changed (I've not yet hunted down the documentation.) In any case, on Build 767, it was generating .tar suffixes and on later builds (newer Python libraries?) it was generating .gtar suffixes. I got burnt by this in Turtle Art so now I check for both.
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if file_path[-5:] == ".gtar" or file_path[-4:] == ".tar":