diff --git a/www/core/components/fileuploader/services/helper.js b/www/core/components/fileuploader/services/helper.js
index e799e7ba7e8..74d25eb75e6 100644
--- a/www/core/components/fileuploader/services/helper.js
+++ b/www/core/components/fileuploader/services/helper.js
@@ -253,6 +253,7 @@ angular.module('mm.core.fileuploader')
             }
 
             if (mimetype && mimetypes.indexOf(mimetype) == -1) {
+                extension = extension || $translate.instant('mm.core.unknown');
                 return $translate.instant('mm.fileuploader.invalidfiletype', {$a: extension});
             }
         }
diff --git a/www/core/lib/fs.js b/www/core/lib/fs.js
index 1af868ab556..3ea4b9c8423 100644
--- a/www/core/lib/fs.js
+++ b/www/core/lib/fs.js
@@ -1599,6 +1599,10 @@ angular.module('mm.core')
      * @since 3.3.2
      */
     self.cleanExtension = function(extension) {
+        if (!extension || typeof extension != 'string') {
+            return extension;
+        }
+
         // If the extension has parameters, remove them.
         var position = extension.indexOf('?');
         if (position > -1) {