add_filter( 'sanitize_file_name_chars', 'turkce_filter', 10, 2 );
 function turkce_filter ( $special_chars, $filename_raw ) {
    $chars = array('ç', 'Ç', 'ğ', 'Ğ', 'ı', 'İ', 'ö', 'Ö', 'ş', 'Ş', 'ü', 'Ü');
    $special_chars = array_merge( $special_chars, $chars );
    return $special_chars;
 }