How Orange makes decision about declaration of a meta attribute
As far as I know, Orange supports advanced annotation with a three-line header in both CSV and Excel files. So if you have a discrete attribute that gets marked as string, you should annote it as discrete on the second header line, or prefix the attribute name on the first line with D#.

When no advanced annotation is provided, it seems Orange's heuristic treats attributes containing general strings (as opposed to numbers) as discrete only if they contain fewer than approximately $\sqrt{|data|}$ unique values.