diff --git a/core/modules/update/src/Tests/UpdateUploadTest.php b/core/modules/update/src/Tests/UpdateUploadTest.php
index 92df4b5..98529b9 100644
--- a/core/modules/update/src/Tests/UpdateUploadTest.php
+++ b/core/modules/update/src/Tests/UpdateUploadTest.php
@@ -127,6 +127,17 @@ public function testUploadModule() {
   }
 
   /**
+   * Ensures that archiver extensions are properly merged in the UI.
+   */
+  function testFileNameExtensionMerging() {
+    $this->drupalGet('admin/modules/install');
+    // Make sure the bogus extension supported by update_test.module is there.
+    $this->assertPattern('/Supported file types:.*update-test-extension/', "Found 'update-test-extension' extension.");
+    // Make sure it didn't clobber the first option from core.
+    $this->assertPattern('/Supported file types:.*tar/', "Found 'tar' extension.");
+  }
+
+  /**
    * Checks the messages on update manager pages when missing a security update.
    */
   function testUpdateManagerCoreSecurityUpdateMessages() {