How do I default the visual studio file property "custom tool" for a given file extension?

I have a custom tool "NafestisGenerator" of which I need to apply to every ".ntf" file. The question is, how do I tell visual studio (using开发者_StackOverflow中文版 the Custom tool Generator sample from Microsoft) to use the "NafestisGenerator" custom tool on every ".ntf" file?

---

I'm pretty sure (87%) that you do this in Windows Explorer.

I notice that if I double-click a .DOC file in my project, it fires up Microsoft Word. Similarly, double-click on an .XLSX file and it fires up Excel.

So, register .NTF as an extension that your tool handles. Then, when you double-click on an .NTF file in VS, it will fire up your tool.