Each product can use either "Single File Download" OR "Package files from other products", but not both -- i.e., you cannot have a product offering file(s) from other products and also offering a file of its own.
If you upload a file for a product with Single File Download enabled, and then later enable "Package files from other products" (which disables the Single File Download setting), then the previously-uploaded file will be unavailable to send for that product and will also be unavailable to package with other products. "Package files from other products" can only be used to "bundle" files from other products which have Single File Download enabled.
