accessing files outside of /home in snap apps

Solution 1:

what is the intended method for using snap apps to work with files stored outside of /home?

The snap should be designed to do that under confinement by making use of the removable-media interface. Connecting such an interface will grant confined access to drives mounted in /media as well as /mnt.

To see if the snap in question supports this, run snap connections <snap name>. For example, for Nextcloud:

$ snap connections nextcloud
Interface        Plug                       Slot           Notes
network          nextcloud:network          :network       -
network-bind     nextcloud:network-bind     :network-bind  -
removable-media  nextcloud:removable-media  -              -

Since nextcloud:removable-media has no associated slot, that means the interface is not connect, thus Nextcloud cannot access removable media. To connect it, we can run sudo snap connect nextcloud:removable-media. Then the connections look like the following:

$ snap connections nextcloud
Interface        Plug                       Slot              Notes
network          nextcloud:network          :network          -
network-bind     nextcloud:network-bind     :network-bind     -
removable-media  nextcloud:removable-media  :removable-media  manual

Now that the plug is connected to a slot, Nextcloud now has confined access to removable media in /media/ and /mnt. Other applications will work the same way if the developer built such functionality in. If they didn't, log a bug! You can find where to log a bug with snap info <snap name>, for example:

$ snap info nextcloud
<snip>
contact:   https://github.com/nextcloud/nextcloud-snap
<snip>