Retaining Upload/Download Files

My workflow has two show form steps (same form) that allows users to upload/download multiple excel files. I can route to the next show form step or save/close the form retaining the download files. But, when I return the form to the originator form step or route it after the initial save step, the files are gone. In addition, when I save/close the form, reopen it, and upload an additional file, the saved download files are replaced by the new upload file(s).
How do I retain the files through multiple routing steps and retain existing saved files when uploading additional files?