It is drop-down list box to allow you to choose from historical entries (the same as address box in web browsers). To imclude more entries, separate then with semicolon. You may see that in default value. Or in documentation.

Thanks for making WinSCP available. It does just what I needed. As a new user of WinSCP, I have a suggestion.
To change the file extensions associated with transferring in text mode, I went to Transfer following files in text mode on the Preferences/Transfer tab. Since this appears to be a drop-down list box, I didn't try editing in it. I searched the documentation to try to find out how to change the settings in that box. Of course, I eventually realized that I could edit that line.
My first thought is that this should be a text box instead of a list box, since there is only one entry in the box. However, there are probably ways of adding other entries to the box. Perhaps the tab could have a line added to it that says something like "Click in the text to change it".
Chris