property FileName: String;

FileName is a TFileName property that represents the file name where the message part is stored. 

When receiving a message, FileName contains the attribute parameter value from the Internet Message header: 

   "Content-Dispositon: attachment; filename=".

FileName is updated during receipt and processing of the message part in TIdMessageClient.ReceiveBody. 

When creating a message for transmission, FileName indicates the local file name where the contents for the message part can be located. FileName is used when the message part is encoded for transmission in TIdMessageClient.SendBody.