can't upload pptx files

I get an error, "The file contents (application/x-zip) do not match its extension (pptx)." The UNIX file command is reporting the mimetype as "application/x-zip". On a different server, running Ubuntu 11.04, it lets the file through, but only because the MimeDetect magic file doesn't work:

file: File 5.4 supports only version 7 magic files. `modules/mimedetect/magic.mgc' is version 4

If I point MimeDetect at the system's magic file, I get the same error as on the other server.

There are a number of similar issues open here, but none of the things that people talk about doing in any of them help - adding an entry to /etc/mime.types makes no difference to the output of the file command, for instance.