diff --git a/core/modules/system/src/Tests/File/NameMungingTest.php b/core/modules/system/src/Tests/File/NameMungingTest.php
index cf3fd89..30bc9e1 100644
--- a/core/modules/system/src/Tests/File/NameMungingTest.php
+++ b/core/modules/system/src/Tests/File/NameMungingTest.php
@@ -71,6 +71,9 @@ function testMungeIgnoreInsecure() {
    * White listed extensions are ignored by file_munge_filename().
    */
   function testMungeIgnoreWhitelisted() {
+    // Most commonly,  extensions are same casing and we expect no changes.
+    $munged_name = file_munge_filename($this->name, $this->badExtension);
+    $this->assertIdentical($munged_name, $this->name, format_string('The new filename (%munged) matches the original (%original) when casing of the extension is same as the whitelisted.', array('%munged' => $munged_name, '%original' => $this->name)));
     // Declare our extension as whitelisted. The declared extensions should
     // be case insensitive so test using one with a different case.
     $munged_name = file_munge_filename($this->nameWithUcExt, $this->badExtension);