Cannot install Windows 7 64bit with Bootcamp
Apparently, Apple is in the process of changing how Windows is installed on Mac's. So, what was true 6 weeks ago may not be true at the moment. Also what is true today, may disapper tomorrow.

VirtualBox can be used as a substitute for the Boot Camp Assistant. There two differing philosophies regarding how to do so. Both install using a legacy BIOS and MBR partition table. 

The first method can be outlined as follows:


Create FAT formatted partition to install Windows. This also creates
a MBR partition table.
Instruct VirtualBox to install Windows into the partition.
Boot Windows in the virtual world and run sysprep. This prepares
Windows to run in the physical world.
Exit VirtualBox and boot Windows.
Install the Boot Camp Support Software.


The actual steps for for Windows 7 are given here. This has been used successfully on Mac's older than 2012. I am not sure about the 2012 model, because the Boot Camp Support Software has the folder $WinPEDriver$ and the file AutoUnattend.xml. You may need the next method.

The second method uses VirtualBox to simulate a Windows PE. Windows is never installed or executed in the virtual world. The outline follows.


Create FAT formatted partition to install Windows. This also creates
a MBR partition table.
Copy the files on the Windows iso to this partition.
Copy the Boot Camp Support Software to this partition.
Create a small image file to hold the MBR bootstrap code.
Boot from the Windows iso using VirtualBox and run Windows commands
modify both partitions. Quit VirtualBox.
Copy the MBR bootstrap code to the MBR of the internal drive.
Boot the Windows partition and install windows.
Delete the iso files and the Boot Camp Support files. (optional)


I don't know if these steps are given anywhere. The closest would be here for Windows 10. A slightly differ version for windows 7 is given here