Haiku considers OpenXML files (docx, pptx, xlsx) to be zip files

Any, I wont bet on it.
But most, yes.

Did the file type was previously identified already?
Because the MIME sniffing doesn’t run on files which have already a MIME type, even if it was guessed a wrong type.

To enforce a new, fresh, MIME guessing:

rmattr BEOS:TYPE <fichier>
mimeset -f <fichier>

Or via Identify from Tracker contextual menu. But first the file type must be removed, if any.

If it still doesn’t work, open your docx with DiskProbe and search for “word/” string pattern and report here the file offset of first match. If there is one…

Wait, did the file is on a non BFS volume?