To expand GotToBTru's response: The SO post you linked hints that the StackOverflow-OriginalPoster had somehow create a modern (.xlsx) spreadsheet with the old (.xls) extension. Thus, I'm guessing the same has happened to you (or the reverse: an old spreadsheet created with the new naming convention).
So, here are some possible routes to go down:
Hope this helps
In reply to Re^2: Reading Excel with different format
by pryrt
in thread Reading Excel with different format
by 9mohit2
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