Index: src/wp-admin/includes/file.php
===================================================================
--- src/wp-admin/includes/file.php	(revision 25025)
+++ src/wp-admin/includes/file.php	(working copy)
@@ -667,7 +667,7 @@
 	global $wp_filesystem;
 
 	// See #15789 - PclZip uses string functions on binary data, If it's overloaded with Multibyte safe functions the results are incorrect.
-	if ( ini_get('mbstring.func_overload') && function_exists('mb_internal_encoding') ) {
+	if ( ( ini_get('mbstring.func_overload') & 2 ) && function_exists('mb_internal_encoding') ) {
 		$previous_encoding = mb_internal_encoding();
 		mb_internal_encoding('ISO-8859-1');
 	}