The MIME type is defined as application/x-keepass2 by KeepassXC. https://github.com/keepassxreboot/keepassxc/blob/develop/share/linux/keepassxc.xml KeepassXC installs this icon: https://github.com/keepassxreboot/keepassxc/blob/develop/share/icons/application/scalable/mimetypes/application-x-keepassxc.svg
A possibly relevant merge request was started @ https://invent.kde.org/frameworks/breeze-icons/-/merge_requests/37