Hi, @Ada
Got it. At this point, can you please run the following command on your CLI and once completed, confirm that the user can view the attachments uploaded?
sudo chmod -R 750 /opt/mattermost/data
Just so we are clear, this issue is pertaining to the attachments and not the posts, right? The reason why I am clarifying on this is because the posts are not stored in the /opt/mattermost/data directory but rather in the Posts table on the database.