MailMessage Content-Type "multipart/mixed" to "multipart/report; report-type=feedback-report"

I'm updating our in-house security software to send XARF reports rather than having to use abuse portals(DigitalOcean).  The only issue I'm having is that a requirement is to have the mail message Content-Type header be "multipart/report; report-type=feedback-report".  I can't find any way to change the message header.  It's either the content-type of the body, or if I add attachments/alternate views, it's "multipart/mixed".

if (mailMessage.Headers["Content-Type"] != null)
mailMessage.Headers.Remove("Content-Type");
mailMessage.Headers.Add("Content-Type", "multipart/report; report-type=feedback-report");