Upload Files in Chromium which are stored in a Cloud-Storage

This restriction is due to Chromium being installed as a snap. snap applications run in a sandbox, a confined environment. To be able to reach your personal files in your home directory, you could enable the home interface. This obviously decreases the confinement of the browser, and hence security.

The snap of chrome is configured to have a home interface, so you can establish the connection with the command:

snap connect chromium:home :home

Your cloud storage must be mounted under your home directory for this to work. If it does not, then you may work around this by mount binding the directory where they are accessed to a directory under your home directory, as in

mount --bind /<path_to_network_drive> /home/<yourlogin>/<somefolder>

To make this permanent, you can add a line in /etc/fstab.