FGT - 1000F Upgrade failed : "File is not an update file"

Hi,

I must upgrade a Fortigate 1000F from 7.2.10 to 7.4.8.

I receive an error that i've never seen : "File is not an update file".

I'v verified checksum of the file.

I've got the same error when i try to upload the file directly to the gui and if i try the upgrade with fortiguard servers.
The support team send me a firmware image to compare with the file i've downloaded and the files are exactly the same.

Has anyone ever encountered this problem?

Thanks
Simon