MultipartEntityBuilder would not support HttpRFC6532Multipart

case EXTENDED:
    if (ContentType.MULTIPART_FORM_DATA.isSameMimeType(ContentType.MULTIPART_FORM_DATA)) {
        if (charsetCopy == null) {
            charsetCopy = StandardCharsets.UTF_8;
        }
        form = new HttpRFC7578Multipart(charsetCopy, boundaryCopy, multipartPartsCopy);
    } else {
        form = new HttpRFC6532Multipart(charsetCopy, boundaryCopy, multipartPartsCopy);
    }
    break;