byte[] attachmentData = ...;
String attachmentContentType = ...;
String attachmentFileName = ...;

MimeBodyPart attachmentPart = new MimeBodyPart();
attachmentPart.setFileName(attachmentFileName);
attachmentPart.setDisposition(Part.ATTACHMENT);
DataSource src = new ByteArrayDataSource(attachmentData, attachmentContentType);
DataHandler handler = new DataHandler(src);
attachmentPart.setDataHandler(handler);