When exporting in mixed mode there should be a place holder in the images for the docs in native mode.
A mixed mode production export is composed of some documents in their native state and image representations of other documents in another subdirectory. The issue is that there is no place holder (slipsheet) in the image directory for those docs that are exported in the native state. This means that there are some production numbers that are missing from the images directory. This idea is to have an option where you can have a slipsheet or an empty file for the docs that are presented in the native form.