Uploading documents as attachments to a question is not available to buy side users.
Buy side users (Question Drafters and Question Submitters) cannot upload new documents when asking a question. They can only link to existing documents or folders they already have access to in the Data Room.
A Guest user can only view the available information in the Data Room. Guests do not get permissions to change anything in the Data Room.
Only users on the sell side (Answer group), can upload documents.