Media: Add flac to the list of allowed file types.

Props blobfolio.
Fixes #42225.

git-svn-id: https://develop.svn.wordpress.org/trunk@42451 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/functions.php

@@ -1291,6 +1291,16 @@ class Tests_Functions extends WP_UnitTestCase {
 							'proper_filename' => false,
 						),
 					),
+					// FLAC file.
+					array(
+						DIR_TESTDATA . '/uploads/small-audio.flac',
+						'small-audio.flac',
+						array(
+							'ext'             => 'flac',
+							'type'            => 'audio/flac',
+							'proper_filename' => false,
+						),
+					),
 				)
 			);
 		}