hi i have this trouble and i fix it by change some rows in \plugins\mail\application\controllers\MailController.class.php
in my 2.6.1 FO first row 2386
//$fName = iconv_mime_decode($att["FileName"], 0, "UTF-8");
//$fName = str_replace(':', ' ', $fName);
$fName = $att["FileName"]; //new line without decode