Sad seems that isn't working anymore in Version 1.10.
Baseline (Widescreen) Solution & Issues
Graphics options start with the Aspect set to "Auto". This autodetects your native aspect ratio, and offers those resolutions. You can manually select a different aspect ratio and resolution. You must manually select the Aspect Ratio first.
If you want the proper FOV in 16:10, you must edit the GameProfile.xml file. If you don't edit the file, the game will be letterboxed on top of the 16:9 FOV. This is only needed for any FOV "less than" 16:9. Using this setting in 21:9 or Eyefinity/Surround will result in Vert- gameplay.
c:\(User)\My Documents\My Games\Far Cry 4
Change the value of UseWidescreenFOV from 1 to 0.
UBISOFT why have you disabled that ?