multipartFile method
Implementation
MethodWithBody multipartFile(String field, List<int>? value,
    {String? filename, MediaType? contentType}) {
  if (_body is! Map<String, Multipart>) {
    _body = <String, Multipart>{};
  }
  if (value != null) {
    _body[field] =
        MultipartFile(value, filename: filename, contentType: contentType);
  }
  return this;
}