Considering that
plasma-discover --local-filename file://$HOME/Downloads/naps2-7.4.0-linux-arm64.flatpak
works, I’m surprised that
plasma-discover --local-filename https://github.com/cyanfish/naps2/releases/download/v7.4.0/naps2-7.4.0-linux-x64.flatpak
doesn’t — I’m aware that it’s local-filename rather than external-filename or merely filename, but it’d be massively more convenient for it to be able to get packages this way too.
sudo zypper install https://github.com/cyanfish/naps2/releases/download/v7.4.0/naps2-7.4.0-linux-x64.rpm
allows it.