Client for multipart/mixed or multipart/form-data endpoints?

[retrofreak83]

Dear all,

I am working on a system which needs to communicate with a REST API I have an OpenAPI spec for. One endpoint is responding in multipart/mixed (they also provide another one which returns multipart/form-data).

The client code is generated fine. However, when the client code is called to access that multipart-endpoint, an exception like this occurs:
Jakarta.ws.rs.ProcessingException: RESTEASY003145: Unable to find a MessageBodyReader of content-type multipart/form-data:boundary=MULTIPART_BOUNDARY and type class...

As REST client dependency, we use quarkus-rest-client-jackson, so according to these sites:

https://quarkus.io/guides/rest-migration#dependencies-2
https://quarkus.io/guides/rest-client

we are using the current REST client variant of Quarkus, formerly known as "RESTEasy Reactive".
However, the documentation of the quarkus-openapi-generator plugin calls that dependency "RESTEasy Classic"
https://docs.quarkiverse.io/quarkus-openapi-generator/dev/client.html

In contrast to that, Quarkus documentation calls
quarkus-resteasy-client "the REST Client compatible with RESTEasy Classic":
https://quarkus.io/guides/resteasy-client

What surprises me is that in some places in all this documentation it is mentioned how you can write a REST server that offers endpoints with multipart responses. It is also explained how you can send a multipart request with a REST client, but there is nothing about receiving a multipart-response.

So, I am wondering whether

• multipart client is possible with API code generated by quarks-openapi-generator at all?
• if so, which REST client variant would be needed?
  And, which additional (glue) code/annotation(s) would be needed.
  While searching for a solution, I found basically information like:
• works automagically
• Annotate API with @RegisterProvider(MimeMultipartProvider.class)
• the model class needs to have appropriate @PartType and/or @FormParam annotations

Any hints are appreciated :)