diff --git a/APPL/Shared/BijlagenPreview.asp b/APPL/Shared/BijlagenPreview.asp
index c8ec1f8209..74f5ec506b 100644
--- a/APPL/Shared/BijlagenPreview.asp
+++ b/APPL/Shared/BijlagenPreview.asp
@@ -79,7 +79,7 @@ function OpenFlexFile(module, niveau, key, kenmerk_key, fname, params)
           for (; !fc.atEnd(); fc.moveNext())
           {
               // Only preview the extensions as specified in S("flexPreviewExt")
-              extension = fso.GetExtensionName(fc.item()); // Extension
+              extension = fso.GetExtensionName(fc.item()).toLowerCase(); // Extension
               if (regex.test("."+extension))
               {
                   lijst.push(fc.item().Name);