When you select one or more multipart messages SMM will save each part of that multipart message as a separate file. Using the file name you provided (or the JMS MessageId when you selected multiple files) SMM will ad a suffix to the message filename with the partnumber. SMM can also save multipart message to .esbmsg format, to do this just enter .esbmsg as the file extension or select the "ESB Message" file type in the save as dialog box.