diff --git a/tests/phpunit/tests/functions.php b/tests/phpunit/tests/functions.php
index 9939678366265..41ce50e90e575 100644
--- a/tests/phpunit/tests/functions.php
+++ b/tests/phpunit/tests/functions.php
@@ -1147,12 +1147,21 @@ public function test_wp_get_ext_types() {
 
 	/**
 	 * @ticket 35987
+	 * @ticket 65819
+	 * @covers ::wp_ext2type
 	 */
 	public function test_wp_ext2type() {
 		$extensions = wp_get_ext_types();
 
 		$this->assertNotEmpty( $extensions );
 
+		// Literal mappings stay independent of the current registry contents.
+		$this->assertSame( 'image', wp_ext2type( 'jpg' ) );
+		$this->assertSame( 'image', wp_ext2type( 'PNG' ) );
+		$this->assertSame( 'audio', wp_ext2type( 'mp3' ) );
+		$this->assertSame( 'video', wp_ext2type( 'mp4' ) );
+		$this->assertSame( 'document', wp_ext2type( 'pdf' ) );
+
 		foreach ( $extensions as $type => $extension_list ) {
 			foreach ( $extension_list as $extension ) {
 				$this->assertSame( $type, wp_ext2type( $extension ) );