If you're running a firmware version below 4.0-Beta7 and you access the unit via HTTPS there's a chance you can receive an error message during firmware upgrade. When you go to the System tab, then upload the file, while uploading it shows an alert of "Waiting for device", then get stuck in "checking file" and don't finish upload process. This is caused by a limitation in HTTPS transfers in firmware version below 4.0-Beta7 which are slower than HTTP. Basically producing a timeout error in the communication between the unit and the computer where the firmware file is being uploaded. This limitation mainly affects firmware upgrade in AF-11FX radios, but could also affect other models.
A workaround is to disable HTTPS and perform firmware upgrade via HTTP.
In firmware 4.0-BETA7 (and above) this limitation has been solved, and there's no need to disable HTTPS for firmware upgrade.