the lib_nonajax.php library in the cforms 10.4 plugin writes wrong mime header lines for attachments at line 479. my corrected version looks like this:
$attached .= '——MIME_BOUNDRY_main_message' . $eol;
$attached .= 'Content-Type: '.$mime.';'.$eol."\\t".'name="' . $n . '"' . $eol;
$attached .= 'Content-Transfer-Encoding: base64' . $eol;
$attached .= 'Content-Disposition: inline;'.$eol."\\t".'filename="' . $n . '"' . $eol;
$attached .= $eol . $file[data]; ### The base64 encoded message
some email clients can handle this issue, outlook 2003 f.e. not.