Add file extension to preprocessor

Author: mfeckie

lib/typescript-preprocessor.js

@@ -15,6 +15,7 @@ class TypeScriptPreprocessor {
   constructor(options) {
     this.name = 'ember-cli-typescript';
     this._tag = tag++;
+    this.ext = 'ts';
 
     // Update the config for how Broccoli handles the file system: no need for
     // includes, always emit, and let Broccoli manage any outDir.