code: public String getContentId() {
          String[] values = getMimeHeader("Content-ID");
          if (values != null && values.length > 0)
              return values[0];
          return null;
      }
docs: Gets the value of the MIME header whose name is "Content-ID".
  
  @return a <code>String</code> giving the value of the
  "Content-ID" header or <code>null</code> if there
  is none
  @see #setContentId