Adding a Document to a Basic Board

I can add a document to a connected card. I don’t think you can add it to the imaginary cards though. 

My understanding is that (currently) boards have no file storage location of their own, and documents can only be stored in projects. So as @skyehansen said, if you have a connected card, you can add a document (since the file is stored in the project). But if you have a non-connected card, then there's no place to store the document, so you can't attach a document.

I'll point you to my own similar question last summer: Kanban Boards and documents.