Affects: 5.13.15
``` public PartBuilder part(String name, Object part, @Nullable MediaType contentType) { Assert.hasLength(name, "'name' must not be empty");
This is wrong and makes no sense for contents like `multipart/mixed`
Field names are only relevant for `multipart/form-data`
c.f. https://www.w3.org/Protocols/rfc1341/7_2_Multipart.html
**Comment From: poutsma**
>This is wrong and makes no sense for contents like multipart/mixed
Opening with a snarky comment like that does not help in getting attention drawn to your issue. If anything, the reverse is true.
`MultipartBodyBuilder` was not designed to be used for `multipart/mixed`, but for `multipart/form-data` where each part gets a `Content-Disposition` header with the part name. Given this context, I would argue that the method signature is correct and makes a lot of sense.
I see that the Javadoc of `MultipartBodyBuilder` can be improved to make this limitation clearer, and I will do so.
**Comment From: jakub-bochenski**
Oh, nice, so WebClient now officially doesn't support `multipart/mixed`
**Comment From: rstoyanchev**
There is some additional context on this.
Our multipart support has evolved historically from multipart form data requests, so it is rooted in that. We added "multipart/mixed" and "multipart/related" as supported media types to the codecs and message converters with #23159 and #23209, but in retrospect that was inadequate.
In any case, this would be a deeper change, most likely not involving `MultipartBodyBuilder` at all, since that prepares a `MultiValueMap`, and hence assumes parts have names.
It would help if you provide a bit more context around your case. Do you need both client and server support, or do you make requests to a 3rd party API? Considering the Servlet API also assumes parts have names, there can be more challenges.
**Comment From: jakub-bochenski**
I'm using a 3rd party API that is receiving and responding with multipart/mixed. So I'm interested just in client support.
I'm able to send multipart/mixed using `MultiValueMap` just by setting all the names to empty string.
But `MultiValueMap` doesn't support "reactive" elements the way `MultipartBodyBuilder` does.
As for reading the response it seems RestTemplate can only write multipart/mixed, not read it.
For WebClient the best I could come up with is
.retrieve() .toEntityFlux((inputMessage, context) -> partReader .read(ResolvableType.forType(byte[].class), inputMessage, context)) ```
but it's inconvenient to the have to deal with raw bytes from DataBuffer for each part.
Also I don't see how would it handle reading multipart nested in multipart --it's a real-world scenario
Comment From: poutsma
I have created https://github.com/spring-projects/spring-framework/issues/30230 to see how much interest there is in the community for Spring to have proper multipart/mixed support.