fix a bug when content of attachment is None (strange bug with email for...

fix a bug when content of attachment is None (strange bug with email forward)

I should fix the problem by allowing attachments like this one but I ddon't have time right now 

--_004_AED2489C945A0F43BAB44072BC97FA211A64EFECTOREXCHANGE01co_
Content-Type: message/rfc822
Content-Disposition: attachment;
	creation-date="Thu, 20 Mar 2014 19:26:43 GMT";
	modification-date="Thu, 20 Mar 2014 19:26:43 GMT"
....