The spec doesn't currently define how binary images are stored in a thick bundle. For example, It would be good to allow images to be stored as single files (e.g. .tar), for simplicity, or as a graph of layers, to optimise bundle size.

duffle seems to create an artifacts directory and corresponding JSON file in a thick bundle, but the spec doesn't mention either of these or how they might be made available to the invocation image(s).

Perhaps the simplest sufficient spec would be to allow arbitrary files and directories inside an optional artifacts directory in the root directory of a bundle and to mount the artifacts directory at a defined location, such as /cnab/app/artifacts, for invocation image(s) to access.