Missing Attachment in Outlook but present in Web Gmail

While coding send mail functionality, found that the attachment functionality is not working properly. The attachment is displayed in the Web browser Gmail / Mozilla Thunderbird but when tested with Microsoft Outlook, Apple Mail for a strange it didn't showed any attachment. After a long debug we found that instead of

Content-Type : multipart/related;

it should be

Content-Type : multipart/mixed;

I am not sure what exactly the difference but it worked out to be a wonder for us. So Please make sure that whenever Attachment is attached thru coding we should use only

Content-Type : multipart/mixed;

Hope this might help you whenever you face this kind of problem.