Can you try editing file 'application/controllers/MailController.class.php' and change line 935 from:
//Classify attachments
$this->classifyFile($classification_data, $conv_email, $parsedEmail, $validWS, true, $csv);
to:
if ($conv_email->getHasAttachments()) {
//Classify attachments
$this->classifyFile($classification_data, $conv_email, $parsedEmail, $validWS, true, $csv);
}