If you're adding an attachment to a document, it will NOT be a binary field, it will be a GUID that is referencing an attachment object which will store the binary data. Try something like this after your DocumentHelper.InsertDocument
call:
AttachmentInfo newAttachment = DocumentHelper.AddUnsortedAttachment(newNode, Guid.NewGuid(), filePath, tree, ImageHelper.AUTOSIZE, ImageHelper.AUTOSIZE, ImageHelper.AUTOSIZE);
newNode.SetValue("FileAttachment", newAttachment.AttachmentGUID);
DocumentHelper.UpdateDocument(newNode);
newNode.Publish();