code: public void attachPlainText(String filenameOnHeader, InputStream resourceStream, String textEncoding) {
          assertArgumentNotNull("filenameOnHeader", filenameOnHeader);
          assertArgumentNotNull("resourceStream", resourceStream);
          assertArgumentNotNull("textEncoding", textEncoding);
          doAttach(filenameOnHeader, "text/plain", resourceStream, textEncoding);
      }
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