title: Altering a multipart/XXX content type without altering the underlying parts
body: <p>I have an instance of MimeMessage which contains encrypted Parts.</p>
  
  <p>The original content type is "multipart/encrypted; protocol="application/pgp-encrypted"; boundary="EncryptedBoundary12312345654654"</p>
  
  <p>After decryption of each parts, I want the multipart header to change as: </p>
  
  <pre><code>"multipart/mixed; boundary="EncryptedBoundary12312345654654"
  </code></pre>
  
  <p>The boundary number is obviously dynamic, then I cannot just make </p>
  
  <pre><code>mime.setHeader("Content-Type", "multipart/mixed;" );
  </code></pre>
  
  <p>Do you have an idea about the best practice for that case? </p>