Firmware upgrade

We manage hundreds of Fortigates remotely and need a solution for quickly upgrading the firmware on FortiAPs. The only options I can find are using a local TFTP server (not an option) or manually logging into the Fortigate GUI and uploading the firmware. Our preference is an option that we can automate, either upgrading automatically or scripting a command from the CLI. Unfortunately I can't find anything like this in any documentation. Please advise.