Function file_save_upload() trims leading non ASCII characters in file name

Hello,

Function file_save_upload() uses basename() function for file name extraction:
$file->filename = file_munge_filename(trim(basename($_FILES['files']['name'][$source]), '.'), $extensions);
But basename() function isn't UTF-8 safe and basename() is returning blank results for strings with multibyte characters (http://php.net/basename).
And it makes it impossible to work with files like 'тест.jpg' (with no ASCII symbols) - Drupal can not identify file extension, filename becomes 'jpg.', uploaded images are not shown and so on.
I think, that file name is a user input and it should not influence on file processing.