It is technically possible to develop an .xll file for Excel on Linux (And I suspect Unix), but it requires specially crafted code for excel to load it. It's also a lot less likely for users on that platform to have Excel installed. So the impact would be not significant. Also executable files definition on Unix systems is different, as it doesn't just depend on the file extension. So I don't think it's worth spending time on those verifications on Unix.
Bug 1843758 Comment 25 Edit History
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It is technically possible to develop an .xll file for Excel on Linux (And I suspect Unix in general), but it requires specially crafted code for excel to load it. It's also a lot less likely for users on that platform to have Excel installed. So the impact would be not significant. Also executable files definition on Unix systems is different, as it doesn't just depend on the file extension. So I don't think it's worth spending time on those verifications on Unix.