Comment 2 for bug 1672436

Just as an added use case (also raised privately), I've also used the state machine options to *delete* files before the image was baked. Probably not a great use case, but there it is.

Still, I think a common feature in both cases is the need to manipulate the contents after `snap prepare-image` has run. There could be two hook points here, one that runs on the unpacked contents before they are shuffled around for the images, and a second one that runs after the shuffling but before the disk image files are baked.