It would be useful to support more explicitly the case of creating an HttpEntity in order to provide a part with headers in multipart request, e.g. a PartHttpEntity sub-class. Such a class could provide a builder similar to those in RequestEntity but by comparison the use case here can make the assumptions and for example a body can always be expected along with a content type.
This was first raised in
SPR-16118 (WebTestClient) but would also help for WebClient and RestTemplate scenarios.