Here’s the story behind it, in three acts. Originally, Mathcad (.xmcd files) used a binary format. Then came Mathcad 14–15 , which introduced an XML-based format (.xmcd still, but now a zip of XML). Then Mathcad Prime arrived with a completely different format (.mcdx, also zip-based but structurally different).
That error message — "The requested file is not a valid Mathcad XML worksheet" — is a classic PTSD trigger for anyone who’s used PTC Mathcad (especially version 15 or Prime). the requested file is not a valid mathcad xml worksheet
But the file is a valid Mathcad XML worksheet — just for , not Prime. Here’s the story behind it, in three acts