code: protected static String[] getMultipartDataBoundary(String contentType) {
          // Check if Post using "multipart/form-data; boundary=--89421926422648 [; charset=xxx]"
          String[] headerContentType = splitHeaderContentType(contentType);
          final String multiPartHeader = HttpHeaderValues.MULTIPART_FORM_DATA.toString();
          if (headerContentType[0].regionMatches(true, 0, multiPartHeader, 0 , multiPartHeader.length())) {
              int mrank;
              int crank;
              final String boundaryHeader = HttpHeaderValues.BOUNDARY.toString();
              if (headerContentType[1].regionMatches(true, 0, boundaryHeader, 0, boundaryHeader.length())) {
                  mrank = 1;
                  crank = 2;
              } else if (headerContentType[2].regionMatches(true, 0, boundaryHeader, 0, boundaryHeader.length())) {
                  mrank = 2;
                  crank = 1;
              } else {
                  return null;
              }
              String boundary = StringUtil.substringAfter(headerContentType[mrank], '=');
              if (boundary == null) {
                  throw new ErrorDataDecoderException("Needs a boundary value");
              }
              if (boundary.charAt(0) == '"') {
                  String bound = boundary.trim();
                  int index = bound.length() - 1;
                  if (bound.charAt(index) == '"') {
                      boundary = bound.substring(1, index);
                  }
              }
              final String charsetHeader = HttpHeaderValues.CHARSET.toString();
              if (headerContentType[crank].regionMatches(true, 0, charsetHeader, 0, charsetHeader.length())) {
                  String charset = StringUtil.substringAfter(headerContentType[crank], '=');
                  if (charset != null) {
                      return new String[] {"--" + boundary, charset};
                  }
              }
              return new String[] {"--" + boundary};
          }
          return null;
      }
docs: Check from the request ContentType if this request is a Multipart request.
  @return an array of String if multipartDataBoundary exists with the multipartDataBoundary
  as first element, charset if any as second (missing if not set), else null