Backup download works fine in old GUI but not in the new GUI

[dnutan (Marc)]

Maybe related to this?

Download openvpn RoadWarrior config not possible in Cockpit with Firefox

We cannot fix download file on backup configuration, but I fixed vpn config download (more used than backup config download ). @giacomo and @davidep will put the fix to testing on Monday. Please @flatspin and @dnutan feel free to test it and report here if it works.