add_filter('sanitize_file_name', 'turkce_filter', 10);
function turkce_filter ($filename) {
 $turkish_chars = array('ç', 'Ç', 'ğ', 'Ğ', 'ı', 'İ', 'ö', 'Ö', 'ş', 'Ş', 'ü', 'Ü');
 $sanitized_chars = array('c', 'C', 'g', 'G', 'i', 'I', 'o', 'O', 's', 'S', 'u', 'U');
 $friendly_filename = preg_replace($turkish_chars, $sanitized_chars, $filename);
 return $friendly_filename;
}