OK I have tried the file again and it also failed but gave me a bit more information stating: 413 Request Entity Too Large
Looking at this similar post Bulk import error from Slack
I attempted to go into mattermost webshell and ran the command (under root user) upon trying to run
sed -i -e 's/50M/5G/' /etc/nginx/conf.d/mattermost.conf
I am left with the response: sed: can't read /etc/nginx/conf.d/mattermost.conf: No Such file or directory
I do have the maximum file size set to: 500000000000 under the system console but wondering if it needs to be adjusted elsewhere as well