add preview handler

ShellExtensions/ThumbnailProviders/ThumbnailProviderAttribute.cs

@@ -1,8 +1,9 @@
-using System;
+#if THUMBNAIL
+using System;
 
 namespace ShellExtensions
 {
-    /// <summary>    
+    /// <summary>
     /// This class attribute is applied to a Thumbnail Provider to specify registration parameters
     /// and aesthetic attributes.
     /// </summary>
@@ -134,3 +135,4 @@ namespace ShellExtensions
         VideoSprockets = 3
     }
 }
+#endif