I've made some progress in understanding how file templates work. The most interesting thing so far was discovering, yesterday, how to sequence categories in the New File wizard. I applied the same principle today while creating the basis for file templates for each of the test cases supported by Cactus. By default, file templates are listed alphabetically. This means that in the screenshot below, the "Java Package" template should come above the "Servlet Test Case" template.
However, by adding this line right above the declaration of the Java Package in the layer.xml file, I specified that it should appear after the "Servlet Test Case" template:
<attr name="ServletTestCase.java/Package.shadow" boolvalue="true" />
In some ways, it's a very small detail. In others, it's pretty cool to know how to do it.