Extension Images # Extension Images are DDIs ( Discoverable Disk Images) that are built to extend a base system via an overlay. A base system or a root DDI can be extended by several extension DDIs via, usually, a read-only OverlayFS. The defining characteristic of an Extension Image is that it contains an extension-release. file that identifies itself and the base system or root DDI it applies to, and must not contain an os-release file.| uapi-group.org