code: public static Encoder getMimeEncoder(int lineLength, byte[] lineSeparator) {
           Objects.requireNonNull(lineSeparator);
           int[] base64 = Decoder.fromBase64;
           for (byte b : lineSeparator) {
               if (base64[b & 0xff] != -1)
                   throw new IllegalArgumentException(
                       "Illegal base64 line separator character 0x" + Integer.toString(b, 16));
           }
           if (lineLength <= 0) {
               return Encoder.RFC4648;
           }
           return new Encoder(false, lineSeparator, lineLength >> 2 << 2, true);
      }
docs: Returns a {@link Encoder} that encodes using the
  <a href="#mime">MIME</a> type base64 encoding scheme
  with specified line length and line separators.
  
  @param   lineLength
  the length of each output line (rounded down to nearest multiple
  of 4). If {@code lineLength <= 0} the output will not be separated
  in lines
  @param   lineSeparator
  the line separator for each output line
  
  @return  A Base64 encoder.
  
  @throws  IllegalArgumentException if {@code lineSeparator} includes any
  character of "The Base64 Alphabet" as specified in Table 1 of
  RFC 2045.