There are two ways to do this. Make something outside of TheBrain and import the file, or create a file from a template (Thought > Attachments > Add Attachment…). One can load up TheBrain with numerous templates -- I have a couple of dozen different file types stored in TheBrain's Template folder in its configuration space and regularly add more. So, for example, if I am working in a portion of a Brain and want to branch off into an external mind map or outline or diagram, from my existing inventory of templates I add a Mind Manager, or OO4 document, or Scapple diagram as an attachment to a Thought. I can click the attachment icon and open that document to edit, and later I can click the icon and use QuickLook (OS X version of TheBrain) to see a preview of that document in a floating window in TheBrain.

Hugh wrote:

At the suggestion of Paul Korm and Steve Z., I've started using TheBrain
as a "graphical file store and manager". So far, it seems to me to be
excellent in that role (except for its expense). I've had no problem
whatsoever in getting files out of it that I've attached to it - for
example,