That expression was default wasn't it? I'd tried that initially with "Read metadata from file contents instead of file name" unchecked.
Just tried it again and it added unknown file name. Is there a setting besides in adding preferences of Calibre that needs to be changed? Pretty sure this is happening when adding though because you can see in the pic (might have to full screen it), the Calibre folder the original Markdown file has "unknown" added and so does the converted text file.
https://www.dropbox.com/s/h68nr5rxsy...%20AM.png?dl=0