[fede01_8]

And what does it exactly restore?

[Jiaz]

It's a normal zip file. You can either open it with an Archive tool or use JDownloader to restore it.
It contains a copy of the complete cfg folder and restores everything (settings,links, accounts). Please note that it does NOT merge but RESTORE(REPLACE).
In case you've got further questions about this, please just ask.
In doubt, please backup/create copy of your current cfg folder first

[djmakinera]

.jd2backup

I do not think it is a complete file and maybe even corrupted. The valid file has the extension .zip (no additional name at the end)