Canon has made EOS 5D Mark III Firmware Version 1.1.3 available for download.
Firmware Version 1.1.3 incorporates the following improvements and fixes.
*1) EF 300mm F2.8L IS II USM / EF 400mm F2.8L IS II USM
*2) Extender EF 1.4X III / Extender EF 2X III
(thanks Niklas, Martin, Sean & Shan)