在Visual Studio 2008中使用非标准扩展名着色文件的语法

I'm probably missing something very obvious here. I have files with the extension .st that contain (mostly) C++ code. I want the C++ in there to be highlighted when I open it with Visual Studio. I go to Tools->Options->Text Editor->File Extension. I type 'st' in the 'extension' box and select 'Microsoft Visual C++' as the editor (I also tried 'source code editor' but that didn't help). But I still don't get syntax coloring. Is there another way?I'm probably missing something very obvious her