Fax attachments download using C#

Hi, I have another problem. Filename is always null. I need filename with extension.
In this case filename is NULL

rc = new RestClient(RingCentralAppKey, RingCentralAppSecretKey,false);
                await rc.Authorize(RingCentralUsername, RingCentralExtension, RingCentralPassword);
                var account = rc.Restapi().Account();
                var extension = account.Extension();
              
                var response = await extension.MessageStore().List();
                var messages = response.records;
                // fax
                var message = messages.Where(m => m.type == "Fax" && m.messageStatus != "SendingFailed" && m.attachments != null && m.attachments.Length > 0);
                foreach (var m in message)
                {
                    var messageResponse = await extension.MessageStore(m.id).Content(m.attachments[0].id).Get();
                    var content = messageResponse.data;
                   
                    var file = new DocumentModel
                    {
                        FileName = m.attachments[0].filename,//this is null
                        FileContent = content
                    };
                }
 

Please check for filename also.