WebClient: Explicit Content-Type application/octet-stream is overwritten

[othi]

Affected version: 6.1.11

The API I'm consuming requires the Content-Type header to be application/octet-stream for certain binary payloads.

The Content-Type is set explicitly using RequestBodySpec.contentType() but it is later overwritten in this method:

spring-framework/spring-web/src/main/java/org/springframework/http/codec/ResourceHttpMessageWriter.java

Lines 165 to 176 in 29dce74

	private static MediaType getResourceMediaType(
	@Nullable MediaType mediaType, Resource resource, Map<String, Object> hints) {
	if (mediaType != null && mediaType.isConcrete() && !mediaType.equals(MediaType.APPLICATION_OCTET_STREAM)) {
	return mediaType;
	}
	mediaType = MediaTypeFactory.getMediaType(resource).orElse(MediaType.APPLICATION_OCTET_STREAM);
	if (logger.isDebugEnabled() && !Hints.isLoggingSuppressed(hints)) {
	logger.debug(Hints.getLogPrefix(hints) + "Resource associated with '" + mediaType + "'");
	}
	return mediaType;
	}

This is unintuitive behaviour and not configurable. To circumvent this, I have to rename the file to an extension not present in spring-web/src/main/resources/org/springframework/http/mime.types so my explicitly set header is respected.

To reproduce, here is a function that sets the content type of the request:

    public void test() throws IOException {
        File f = File.createTempFile("test", ".pdf");

        WebClient webClient = WebClient.create();
        RequestBodySpec builder = webClient.method(HttpMethod.POST).uri("http://localhost:12458");
        builder.contentType(MediaType.APPLICATION_OCTET_STREAM);
        builder.body(BodyInserters.fromValue(new FileSystemResource(f)));
        builder.retrieve().toBodilessEntity().block();
    }

Resulting request using netcat:

me@dev:~$ nc -l 12458
POST / HTTP/1.1
accept-encoding: gzip
user-agent: ReactorNetty/1.1.9
host: localhost:12458
accept: */*
Content-Type: application/pdf   <---------
content-length: 0

Expected result:

Content-Type: application/octet-stream

Thanks