Today you can add files to a document set only if they are in the Plans folder. We also want to add files from the Project Files folders to a document set.
e.g. Our product specifications are in PDF format and can only be stored in the Project Files folder (when we should upload them to the Plans folder, BIM 360 Docs will try to extract titleblock information which does not make sense for textual documents). Now we want to add these specifications in a document set. This is not possible because they are in the Project Files folder.