set_content_type

Used by mail to set the content type of the message.

It will use the given user_content_type, or multipart if the mail message has any attachments. If the attachments are inline, the content type will be âmultipart/relatedâ, otherwise âmultipart/mixedâ.

If there is no content type passed in via headers, and there are no attachments, or the message is multipart, then the default content type is used.