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".