I had a problem with my Flatpak installation of Slack - I couldn't save files.
I don't recall this being an issue before, but in my new computer with Ubuntu Mate 21.10, it was.
Turns out the solution is fairly straightforward.
Flatpak has the option to specify a file path which can then be written to. If this file path is then the default download folder everything is copacetic.
Steps:
Close Slack and make sure it is not running in the background.
Run flatpak ps to ensure com.slack.Slack is not listed.
Create a directory for storing the download files. This can be in your file manager (Nautilus, Caja, Dolphin, Thunar, etc.), or with the mkdir command.
Once created you can run the following:
flatpak override com.slack.Slack --filesystem=<your full directory path>
*This command needs to be run as root in my testing (use sudo).
Then start the Slack application again and go to Preferences -> Advanced -> Download location. Set it the same as the directory path just mapped.
That's it. Files should now be able to be saved to that specific directory.
Hope it helps anyone who needs to do something similar in Slack or another Flatpak application.