Prior to this commit, RestTemplate posted multipart with Content-Type
"multipart/form-data" even if the FormHttpMessageConverter configured
in the RestTemplate had been configured to support additional multipart
subtypes. This made it impossible to POST form data using a content
type such as "multipart/mixed" or "multipart/related".
This commit addresses this issue by updating FormHttpMessageConverter
to support custom multipart subtypes for writing form data.
For example, the following use case is now supported.
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("https://example.com/myFileUpload", requestEntity);
Closes gh-23159
5008423408