Add explicit support for multipart/mixed in FormHttpMessageConverter

[sbrannen]

Overview

gh-23159 added support for multipart/* media types in FormHttpMessageConverter, but users must still manually register multipart/mixed as a supported media type in order to POST multipart data with that content type.

For example, something similar to the following code is required to get it work.

MediaType multipartMixed = new MediaType("multipart", "mixed");

restTemplate.getMessageConverters().stream()
    .filter(FormHttpMessageConverter.class::isInstance)
    .map(FormHttpMessageConverter.class::cast)
    .findFirst()
    .orElseThrow(() ->
        new IllegalStateException("Failed to find FormHttpMessageConverter"))
    .addSupportedMediaTypes(multipartMixed);

MultiValueMap<String, Object> parts = new LinkedMultiValueMap<>();
parts.add("field 1", "value 1");
parts.add("file", new ClassPathResource("myFile.jpg"));

HttpHeaders requestHeaders = new HttpHeaders();
requestHeaders.setContentType(multipartMixed);
HttpEntity<MultiValueMap<String, Object>> requestEntity =
    new HttpEntity<>(parts, requestHeaders);

restTemplate.postForLocation("example.com/myFileUpload", requestEntity);

Since multipart/mixed is rather common in REST scenarios, it would be nice if users could forgo the steps required to manually register it as a supported media type.

Deliverables

• Introduce a multipart/mixed constant in MediaType.
• Add explicit support for multipart/mixed in FormHttpMessageConverter.