session.checkIn() takes mimetype from properties or from options
as the most common use case should be checking modifying the properties received with checkOut:

 cmisSession.checkOut(objectId).ok(function(data) {
        cmisSession.checkIn(
                data.succinctProperties['cmis:objectId'],
                true,
                data,
                updatedContent,
                aComment
	)
    });

fixed #3
updated documentation