Cannot send an MP3 file as email attachment...

I want to send an email with an MP3 file as attachment. If I do not
set content type, the file received will not be in the correct format.
But when I set content type to audio/mp3, I will receive
unsupporteddatatype exception. How can I send an mp3 file as an
attachment so that when I download the mp3 file from the email, the mp3
file will work and play as usual?

Someone said I may modify the activation.jar by editing the mailcap
file. However, I don't know what to add to the file. The file only
contains:

#
# This is a very simple 'mailcap' file
#
image/gif;; x-java-view=com.sun.activation.viewers.ImageViewer
image/jpeg;; x-java-view=com.sun.activation.viewers.ImageViewer
text/*;; x-java-view=com.sun.activation.viewers.TextViewer
text/*;; x-java-edit=com.sun.activation.viewers.TextEditor

My program is below:
=======================
package my;
import javax.mail.*;
import javax.mail.internet.*;
import java.util.*;
//import java.lang.Object.*;
//import javax.activation.CommandMap.*;
//import javax.activation.MailcapCommandMap.*;

public class rSendMail{
    private String to;
    private String from;
    private String smtpServer="smtp.macau.ctm.net";
    private String subject;
    private String body;
    private String contenttype;
    private String attachfile;

  public void setTo(String sTo)
  {
   to=sTo;
  }

  public void setFrom(String sFrom)
  {
   from=sFrom;
  }

  public void setSmtpServer(String sSmtpServer)
  {
   smtpServer=sSmtpServer;
  }

  public void setSubject(String sSubject)
  {
   subject=sSubject;
  }

  public void setBody(String sBody)
  {
    body=sBody;
  }

  public void setContentType(String sContenttype)
  {
   contenttype=sContenttype;
  }

  public void setAttachment(String sAttachment)
  {
   attachfile=sAttachment;
  }

  public void send(String path)
  {
    try
    {

//MailcapCommandMap mc =
(MailcapCommandMap)CommandMap.getDefaultCommandMap();
//mc.addMailcap("text/html;;
x-java-content-handler=com.sun.mail.handlers.text_html");
//mc.addMailcap("text/xml;;
x-java-content-handler=com.sun.mail.handlers.text_xml");
//mc.addMailcap("text/plain;;
x-java-content-handler=com.sun.mail.handlers.text_plain");
//mc.addMailcap("multipart/*;;
x-java-content-handler=com.sun.mail.handlers.multipart_mixed");
//mc.addMailcap("message/rfc822;;
x-java-content-handler=com.sun.mail.handlers.message_rfc822");
//CommandMap.setDefaultCommandMap(mc);

      Properties props = System.getProperties();

      // -- Attaching to default Session, or we could start a new one
--
      props.put("mail.smtp.host", smtpServer);
      Session session = Session.getDefaultInstance(props, null);

      // -- Create a new message --
      Message msg = new MimeMessage(session);

      // -- Set the FROM and TO fields --
      msg.setFrom(new InternetAddress(from));
      msg.setRecipients(Message.RecipientType.TO,
      InternetAddress.parse(to, false));

      msg.setSubject(subject);
  msg.setText(body);
// StringBuffer sb = new StringBuffer();
// sb.append("<a href='"+path+"'>HERE</a>");
// msg.setText(body+sb.toString());

// msg.setContent(attachfile,contenttype);
// msg.setFileName(attachfile);

      // -- Set some other header information --
      msg.setHeader("X-Mailer", "LOTONtechEmail");
      msg.setSentDate(new Date());

      // -- Send the message --
      Transport.send(msg);

// System.out.println("Message sent OK.");
    }
    catch (Exception ex)
    {
      ex.printStackTrace();
    }
  }

}