I've reviewed the file field code and there's no way to substitute just the file URL. It'll always format it with an <a element. You can probably get away with formatting the download URL manually as follows though.
[cb:config param="live_site" /]/index.php?option=com_comprofiler&view=fieldclass&field=FIELD_NAME&function=download&user=[user_id]&reason=profile
Replace FIELD_NAME as needed. Leave the rest to substitute as normally. Have added a bug ticket for this as non-HTML output of the file field should be the file URL only.
forge.joomlapolis.com/issues/5358