Hello, An unknown MIME type in a desktop environment does not prevent a file to be open by an application that can manage it but it prevents double-clicking the file because there is no known prefered application to open it.
The MIME page of the Debian wiki: https://wiki.debian.org/MIME (if you are using Plasma) the KDE file association page: https://docs.kde.org/trunk5/en/kde-workspace/kcontrol5/filetypes/index.html
