Hi John,
Once you add a field with attachment type to the table and then go to the customize screen you'll see 2 fields that can be added to the screen; example, Attach and Attach File Name. In your case please remove the 'File Name' one from the screen. Once this is done then try to add a document and it should work. Just a heads up for our next update, we will be removing the 'File Name' field from the list of available fields to be added to a screen. This way you and others wouldn't run into the same situation in the future :)