Bootcamp Windows 8.1
The downloadable version of Windows 8.1 is upgrade only, not full install, the one with the EXE is for upgrading from Windows 7 or 8... the key being from Windows 8 here.. 

Simple solution, if a little bodgy...

Download the Windows 8 demo here: http://www.soft-zilla.com/2013/01/windows-8-iso-download.html​

Then upgrade to 8.1 from that using our EXE file.