IMail.Visuals are always copied to the reply.
IMail.NonVisuals are copied only if ReplyBuilder.IncludeAttachments property is set to false.
IMail original = ...
ReplyBuilder replyBuilder = original.Reply();
replyBuilder.IncludeAttachments = true;
MailBuilder builder = replyBuilder.Reply(new MailBox("firstname.lastname@example.org"));
IMail replied = builder.Create();
As a side note:
Embedded images should be stored as visual elements (Content-Disposition: inline or no Content-Disposition header instead of Content-Disposition: attachment)