Latitude 5414 BIOS update

I'm trying to update the BIOS of a Latitude 5414 Rugged laptop.
Because it's running Ubuntu I can't use the Dell tools and I'm using the steps from "Updating the BIOS on supported UEFI systems".

I've put the latest BIOS version on an FAT32 formatted USB stick and selected BIOS Flash Update from the One Time Boot Menu.
Here I can select the BIOS file, but after loading it the BIOS Flash Update tool gives the following message:invalid BIOS file?System:
Revision:
Vendor:

I've also tired all the versions between 1.22 and 1.34, but they all give the same result.

What can I do to update the BIOS?

Thanks,
Matt