iTunes U downloads Numbers as a .zip file to iOS

I have uploaded six Numbers [v 3.2 (1861)] spreadsheets to an iTunes U course. Five of them download to my iPad [iOS 7.2] and open in Numbers, but one downloads as a .zip file and cannot be opened. This problem was identified by a student, so the same problem exists on other devices. All the spreadsheets were created in Numbers on my MacBook Pro [10.9.4] and uploaded to iTunes from the MacBook.

Any suggestions?