Dear @ahmaddanial,
I would like to inform you that I can be able to see the images and files again as well as download them at Mattermost.
Also, the directory ownership in the /opt/mattermost/ directory looks like show as attachment.
Moreover, I would like to ask whether you can find ways to solve the problems at Mattermost send email notification problems . I have the same problem of using Mattermost at Ubuntu 16.04 LTS. I use the same config.json as above question. Thank you.