Refer to files with unknown extensions as assets that use the default asset importer.

Source/ToolCore/Assets/Asset.cpp

@@ -274,7 +274,7 @@ bool Asset::CreateImporter()
         {
             importer_ = new ModelImporter(context_, this);
         }
-        if (ext == ".ogg" || ext == ".wav")
+        else if (ext == ".ogg" || ext == ".wav")
         {
             importer_ = new AudioImporter(context_, this);
         }
@@ -334,6 +334,10 @@ bool Asset::CreateImporter()
         {
             importer_ = new TextureImporter(context_, this);
         }
+        else
+        {
+            importer_ = new AssetImporter(context_, this);
+        }
 
     }