optimization image check.

public/javascripts/application.js

@@ -625,15 +625,11 @@ $(document).ready(addFormObserversForDoubleSubmit);
 function img_thumbnails() {
   $('.thumbnails a').colorbox({rel:'nofollow'});
   $('.attachments').find('a').each(function(index, element) {
-    if($(element).attr('href').indexOf('.jpg') > 0) {
+    var href_value = $(element).attr('href');
+    if (/\.(jpg|png|gif|bmp)$/.test(href_value)) {
       $(element).colorbox({rel:'nofollow'});
     }
-    if($(element).attr('href').indexOf('.png') > 0) {
-      $(element).colorbox({rel:'nofollow'});
-    }
-    if($(element).attr('href').indexOf('.gif') > 0) {
-      $(element).colorbox({rel:'nofollow'});
-    }
-  })
+
+  });
 }
 $(document).ready(img_thumbnails);