CakeEmail adding Content-Type: multipart/mixed when sending Email in text and html format

I have been using CakeEmail to send email with format text and Html.But In outlook email received from the system shows the attachments icon  But there is no attachments attached to the email.I found taht it si because of In header Content-Type: multipart/mixed; sent but it should be multipart/alternative.I tried to override this by using $Email->setHeaders() function It do not overrirde it..

Please Help If someone knows solution to this issue.I will be really thankful.

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