Comment by jeroenhd

Android's model, where you can ask the user for (read only/read+write) access to only specific files/a specific folder works pretty well for that.

But on Discord's side, this is also because the people repackaging Discord for Flatpak were quite conservative. If you want to break open the sandbox a bit more, you can grab Flatseal and manually approve additional directories. I'm not exactly sure what you need for drag&drop to work, but when I add a folder to Geary's whitelist (or grant all home folder permissions, I suppose), I can drag it into the Flatpak'd application like normal.

Unlike on Android, Flatseal actually lets you list those directories and lets you revoke them (at your own risk).