Hi Ghosty and welcome to the Mattermost forums!
7.8.0 is supposed to have the fix for this issue, but it’s not the latest version, so upgrading to 7.10.3 should still be an option for you to rule out that there’s some other bug that prevents you from uploading files successfully.
Do you have an antivirus plugin running in Mattermost that could be the reason for the stuck uploads?