I'm having issues with this too. I think the documentation is lacking. I don't believe uploading stuff like new firmware applies to us anymore.
The documentation on github states:
You should also upload the new IAP file for your system. You will need it when upgrading firmware in future. These files are called Duet2CombinedIAP.bin, DuetMaestroIAP.bin, Duet3_SBCiap_MB6HC.bin (for Duet 3+SBC)
For the Duet and PI combination, you would believe that we should use Duet3_SBCiap_MB6HC.bin
But it's not one of the files in the releases.
I had been told that the updated firmware is installed when you update the duetsoftwareframework package on the pi using the unstable branch on the package server (since it is RC)
Which did successfully update to the most current at the time, which was 3.0RC1
But now that RC2 is out, I'd like to update to it and when I go to update duetsoftwareframework it states it's already up to date.
I'd like cleared documentation on how to actually upgrade the firmware on a Duet+SBC machine.
For your issues, to get you a little closer,
SSH into your pi and and run these to get the unstable repository
wget -q https://pkg.duet3d.com/duet3d.gpg
wget -q https://pkg.duet3d.com/duet3d-unstable.list
sudo mv duet3d.gpg /etc/apt/trusted.gpg.d/
sudo mv duet3d-unstable.list /etc/apt/sources.list.d/duet3d-unstable.list
sudo chown root:root /etc/apt/trusted.gpg.d/duet3d.gpg
sudo chown root:root /etc/apt/sources.list.d/duet3d-unstable.list
apt-get upgrade duetsoftwareframework
Edit: in your original post
Instead of doing it via the web interface, maybe put it into that location on the pi directly?