How to create Windows 10 installation USB for a Mac without Bootcamp Assistant?

I'm trying to install Windows 10 x64 on a Mac Pro (3,1). The Mac doesn't have OS X installed so I don't have access to Bootcamp Assistant. I tried the following:

• Windows 7 USB DVD Download Tool
  
  
  
  
  • creates a NTFS USB and doesn't show up in Mac's boot selection screen (pressing option at boot)
  
  



• diskpart + xcopy

  
  
  
  
  
  

  • creates a FAT32 USB. It only shows EFI Boot (the UEFI boot) at the boot selection screen but not Windows (the MBR boot)

    
    
    

    

  
  

Did I miss something obvious here?