Give text files a special template iterator. It would add an optional
third panel. If the user advanced to this panel, there would be some
explanatory text, a combo box, and a checkbox. The combo box would
list all registered text loader extensions, or the user could enter a
new extension. The box's value would default to the extension of the
template file (typically "txt"). If the value (default or entered) was
not in the standard list, the checkbox would become enabled (off by
default). Finishing the wizard would create the file with the
specified extension (marking it as a text file with a file attribute
if necessary). If the checkbox was enabled and checked, the extension
would also be added to the list of registered extensions. Explanatory
text would explain all this, e.g.:

You may select an alternate extension for your new text file. Choose
from standard text-file extensions. Or, you may enter your own. If you
enter a nonstandard one, by default just this file will be treated as
a text file; you may also request that all files with this extension
be considered text files, too. (Tools | Options | Object Types | Text
Files | Extensions also lets you add standard extensions to treat as
plain text files.)