I was trying to upgrade the BIOS on a Asus P8H61-M PRO motherboard on a floppy-less PC.
Luckily this motherboard supports upgrading from the BIOS itself.
The problem was I got this error once the image was selected on a disk partition (strangely enough the BIOS saw only one of the two disks):
Selected file is not an EFI BIOS
It turned out the file should reside on a FAT32 partition while the one it was on was NTFS.
I put the new BIOS on a pen drive and the BIOS saw it, finally ending the story.
Side note: the error message was pretty useless, once again.