Apache Commons Email encode attach with base64

I'm trying to send a file base64-encoded via apache.commons.mail and I just can't seam to get the Content-Transfer-Encoding: base64 header where it's supposed to go.

// Create the email
 MultiPartEmail email = new MultiPartEmail();
 email.setSmtpPort(587);
 email.setDebug(false);
 email.setHostName("smtp.gmail.com");
 email.setAuthentication("from@gmail.com", "password");
 email.setTLS(true);
 
 email.addTo("to@example.com");
 email.setFrom("from@example.com");
 email.setSubject("subject");
 
 email.attach(new ByteArrayDataSource(
      Base64.encodeBase64(attachFull.getBytes()), "text/plain"), 
      "samplefile.txt", 
      "sample file desc", 
      EmailAttachment.ATTACHMENT
 );
 

And this is what gets to the recipient.

------=_Part_0_614021571.1334210788719
 Content-Type: text/plain; charset=Cp1252; name=texto.txt
 Content-Transfer-Encoding: quoted-printable
 Content-Disposition: attachment; filename=samplefile.txt
 Content-Description: sample file desc
 

How I can specify that the file is Base64 encoded?

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