New Post: Mac / Numbers can't read generated file

I'm using ClosedXML to create and download .xlsx files on the fly from my asp.net (c#) webform application. All is well if I use Excel in Windows to read the file but that same file isn't read by Numbers on a Mac. It just opens an empty document with no sheets. A couple of interesting points:

1. If I download the file to a Windows machine, open it, re-save it, then copy to the mac, Numbers will read it fine.
   
2. If I just copy the Windows downloaded file over to the mac without re-saving it, an empty file is shown. No errors other than a font warning are shown by Numbers.
   

The problem exists even with the simple Hello World document created with the code shown here: https://closedxml.codeplex.com/wikipage?title=How%20do%20I%20deliver%20an%20Excel%20file%20in%20ASP.NET%3f

Any ideas or workarounds?