Cannot Open PowerPoint with Office 2013 Web Apps Server

I have installed an instance of Office 2013 Web Apps Server and applied all updates from Microsoft Update. I am in the process of developing a WOPI Server per the specification at http://msdn.microsoft.com/en-us/library/hh622722(v=office.12).aspx.

So far I can open/edit Excel documents just fine, and view Word documents. (I cannot edit word documents because I haven't yet implemented the complex-looking MS-FSSHTTP protocol.) However, whenever I try to view or edit a PowerPoint presentation (pptx), I get the following error message:

"We can't open this presentation because the file format is blocked by your administrative policy."

I see how to get around this problem with the traditional PowerPoint application, but I cannot find any solution to this for the Web Apps version. Any help on what this error means and how I can enable viewing of PowerPoint would be appreciated.

Thanks,

-Richard