Use ReplyBuilder.IncludeAttachments property, by default it is set to false, so no attachments are send back in the reply email:
IMail original = ...
ReplyBuilder replyBuilder = original.Reply();
replyBuilder.IncludeAttachments = true;
MailBuilder builder = replyBuilder.Reply(new MailBox("me@example.com"));
IMail replied = builder.Create();