Charset in accept makes the server ignore the entire media-type requested

[csteeg]

Describe the bug
When sending the charset alongside the media-type in the accept-header, the entire header is ignored.
If sending "Accept: application/fhir+xml;charset=UTF-8" I get a json response.
Sending "Accept: application/fhir+xml" or "Accept: application/fhir+xml;q=0.8" I correctly get xml.

According to the standard https://www.[iana.org/assignments/media-types/application/fhir+xml](https://www.iana.org/assignments/media-types/application/fhir+xml) it's an accepted parameter for this media-type

FHIR Version?
Stu3/R4/R5 -> all

Data provider?
CosmosDB/SQL Server -> both

To Reproduce
Steps to reproduce the behavior:

1. Send a request with the header "Accept: application/fhir+xml;charset=UTF-8"

Expected behavior
An xml result

Actual behavior
JSON result

[EXPEkesheth]

Thanks for reporting the issue!

Are you using the OSS FHIR service or managed FHIR service? Marking it for "UpforGrabs"

[csteeg]

We're using the OSS self hosted version

[EXPEkesheth]

@csteeg - Thanks for the response! The Azure FHIR service supports JSON by default. Currently, XML support is only available as an open-source capability, which makes it a lower priority for our queue at this time.

[csteeg]

This seems to be a problem of .net MVC framework?
If I add
SupportedMediaTypes.Add($"{KnownContentTypes.XmlContentType}; charset=utf-8");
SupportedMediaTypes.Add($"{KnownContentTypes.XmlContentType}; charset=utf-16");
to the FhirXmlOutputFormatter constructor, it works.

The CanWriteResult of the base OutputFormatter, calls parsedContentType.IsSubsetOf(supportedMediaType) which in the end calls private bool ContainsAllParameters(MediaTypeParameterParser setParameters) in ReadOnlyMediaTypeHeaderValue

I don't know exactly if IsSubsetOf is really supposed to match the Charset also, but if so the supportedmediatypes should be added for all currently supported mediatypes I guess