Avirk's answer actually doesn't scan a file or folder directly, it just initiates a quick or full scan of your entire computer.

http://www.thewindowsclub.com/add-scan-with-windows-defender-option-context-menu-windows-8 outlines a method to get the scan to work on folders. I think with some modifications it can be made to work on files as well.

UPDATE:

It is fairly easy to get this to work on files, just change the /folder/ part of the registry keys to /*/.

Windows Registry Editor Version 5.00 [HKEY_CLASSES_ROOT\*\shell\WindowsDefender] "MUIVerb"="Scan With Windows Defender" "Icon"="%ProgramFiles%\\Windows Defender\\EppManifest.dll" @="" [HKEY_CLASSES_ROOT\*\shell\WindowsDefender\Command] @="\"C:\\Program Files\\Windows Defender\\MpCmdRun.exe\" -scan -scantype 3 -SignatureUpdate -file %1"

(Just paste the above in a file with extension.reg and merge it into the registry by double-clicking the file.)

The one thing I can't get working is bringing up the GUI scan versus a console version of the scan output.