private byte[] getMultiPartBoundary(MediaType contentType) {
    val boundaryString = unquote(contentType.getParameter("boundary"));
    if (StringUtils.hasLength(boundaryString) == false) {
      throw new HttpMessageConversionException("Content-Type missing boundary information.");
    }
    return boundaryString.getBytes(UTF_8);
  }
Construct an {@code AbstractHttpMessageConverter} that can read mulitpart/form-data.

@param bufSize The size of the buffer (in bytes) to read the HTTP multipart body.
form-spring/src/main/java/feign/form/spring/converter/SpringManyMultipartFilesReader.java