Android : Adding attachment in android using Java Mail

I am trying to add attachments to an email which I send using JavaMail. I can send the email without attachment but I get an error when I try to attach a file.

03-28 18:07:36.735: E/SendMail(6703): null
03-28 18:07:36.735: E/SendMail(6703): java.lang.NullPointerException
03-28 18:07:36.735: E/SendMail(6703):   at com.example.email.GMailSender.addAttachment(GMailSender.java:95)
03-28 18:07:36.735: E/SendMail(6703):   at com.example.email.MainActivity$1$1.run(MainActivity.java:28)

Here is the method I use to attach a file.

public void addAttachment(String filename) throws Exception {
        BodyPart messageBodyPart = new MimeBodyPart();
        DataSource source = new FileDataSource(filename);
        messageBodyPart.setDataHandler(new DataHandler(source));
        messageBodyPart.setFileName(filename);
        _multipart.addBodyPart(messageBodyPart);  
        message.setContent(_multipart);
    }

and I call it in the Main Activity.

sender.addAttachment("/storage/extSdCard/DCIM/Camera/Photo.jpg");

Do you think there is something wrong with the path of the file, if so how can I get the correct path.