Specifically, for a set of image files, I want to map their aspect ratios - {File.AspectRatio} - into an Attribute (Would data type, set to Text, be best, in this case?), named, "Aspect Ratio". I attempted, to solve the problem, via an MD Template. But, it so appears, that those are irrelevant, for Attributes.
Attribute importer in the Import & Export panel.