Just click on the "File types configuration" link on the left panel of the configuration utility. When this window is shown, the only field that is going to be enabled is "File type" and you will have to select the extension you want to see/modify.

Once you have selected the desired extension the rest of the fields will be enabled. After you finish to modify the settings of EACH extension, click on "apply" button to changes take effect. After the adjusts are done click Ok to save the configuration permanentely.

It is important to remmark that TXT, LST and PRN files are basically the same kind of file: plain files. What we pretend by separating them is to have three different configurations for the same kind of file. For example if your DOS application has to print some reports using a font size and some others using another one, you can just define ones as TXT and others as LST for example. After that you must configure the print server as desired.