Flatpak Communication with non-flatpak data

[ndak]

I installed Gnote-flatpak via Software Mgr in the new Mint 18.3 Cinnamon 64-bit. My question is how do I communicate with non-flatpak data files thru Gnote sync tool? I want to add existing Tomboy notes to Gnote-flatpak but can't access the Tomboy data file from Gnote-flatpak. My impression is that flatpak installations are self-contained and cannot access other parts of the system including data files. Is this correct? Is there any way for flatpak to communicate with other parts of the system?
Thanks,
ndak

[xenopeek]

My impression is that flatpak installations are self-contained and cannot access other parts of the system including data files. Is this correct?

That is partially correct. The sandbox is configured per Flatpak. For some programs, like Gnote, it doesn't make sense to have it be able to access all your files by default (I assume that's why it is configured with this restriction). For example if you install the Gedit Flatpak you'd see it can access your entire home directory, as that does make sense as a default.

You can override the default sandbox configuration for a Flatpak: http://docs.flatpak.org/en/latest/worki ... ermissions

Specifically for Gnote installed from Software Manager you must run this command on the terminal to change the sandbox configuration for Gnote so that it will be allowed to access your home directory:
sudo flatpak override --filesystem=home org.gnome.Gnote
You only need to do this once.

[ndak]

Thanks Xenopeek! Your response answers my questions very well!
ndak