Alright, so this is the live (the quoted line above) that needs to be changed. This is telling the server that by default, if it doesn’t recognize a specific file extension, or it isn’t specified in mime.types then it should download it to the user’s computer. By changing this line to default_type text/html;, and then restarting your Nginx server (sudo systemctl restart nginx) as well as restarting your Mattermost instance (sudo systemctl restart mattermost) you should be good to go!
Let me know how it goes and if it doesn’t work or you encounter any other issues, I’ll be more than happy to help out!