With a school of 550 students and all classes running Moodle in some capacity, I have never had a verified problem (that I could replicate) of uploading such documents. I have heard rumors of a student or two who thought there was a problem, but they said they were successful when they switched browsers (like from IE to Firefox), but I suspect this might be a configuration or cache problem on their end.
The other thing to look for with Word, Excel, or Powerpoint documents is that you don't have an unsaved version of the document open when you try to upload it. Your brower does not tell the application to save the document before doing an upload, so you might get a failed or corrupt upload because it is not in a saved state.
Finally, if you can find a verified and especially repeatable instance of a valid document not uploading, make sure to post the situation to tracker.moodle.org so the problem can be fixed. The version and current update situation of your Moodle software will be an important thing you should report in such situation to try to track down the problem.
--Gary
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