Ok, I have already been able to change the settings, I had to access the directory in /etc/mattermost and edit as I show here:
nexus@nex-Ubuntu:/etc/mattermost$ sudo edit mmomni.yml to be able to modify with vi editor and save changes. After restarting with mmomni reconfigure, I already see the parameter saved in “true” as checked with sudo -u mattermost mmctl --local config get PluginSettings.EnableUploads
Thank you and sorry for not having understood that you had to enter to edit in the directory where mmomni.yml is hosted