I have tested this issue at my end and could observe the issue you have specified. Despite the fact that the result file lacks the multipart, the output EML opens fine in Outlook and there is no visible issue. Could you please share if this has some effect at your end or you get any problem while viewing the message? We’ll look into your requirements from your feedback and discuss any such possibility with our development team (as far now, I couldn’t find any such information to achieve your requirements).
I have logged the issue as NETWORKNET-34469 in our issue tracking system for further consideration by our development team. We’ll notify you here once there is some update available in this regard.