When calling custom items into a page you may choose to name these files with an extension other that tpl. The normal code for calling custom templates will not recognize anything except files with the normal tpl extension. This does not mean that you can not use other extensions for you files, but you will need to add information to your anchor tag in order for it to find this file type.

Here is a normal link:
<a href="http://scripts.webmastersite.net/wsnlinks/wsnmanual410/index.php?action=userlogin">Login</a>

Here is a link calling a custom template:
<a href="http://scripts.webmastersite.net/wsnlinks/wsnmanual410/index.php?action=userlogin&custom=yes&TID=nameof template">Login</a>

Here is a link calling a custom file without an tpl extension:
<a href="http://scripts.webmastersite.net/wsnlinks/wsnmanual410/index.php?action=userlogin&custom=yes&TID=nameof template&ext=txt">Login</a>

Adding the ext= to your anchor tag will allow you to call any type of file into your customization process.