Debian installer - load network firmware

during the installation of Debian the installer asks if I should try to load film where from an external source.

But even if the relevant firmware for one or more of the network adapters network has been placed on some partition that should be available for the installer it fails.

The reason I found was that the partition containing the firmware must be in FAT(16). Yes this is a Linux installer that can not use EXT4 partitions as the container of firmware drivers.

Steps

cd /tmp
apt download firmware-iwlwifi
mkfs.fat /dev/sdXY
mkdir -p /mnt/usb
mount /dev/sdXY /mnt/usb
mv firmware-iwlwifi* /mnt/usb
sync
umount /mnt/usb

Boot the installation and ensure the usb with the driver is connected too.