Hi, Before fileing a Jira-Ticket I wanted to confirm that the behavior I describe below is really a bug in resteasy or works as designed. While implementing REST-Services in a quarkus application we came across the problem that when doing fileuploads who are require the use of resteasy-multipart-provider we can not use custom converters registered for our POJOs (well in our REST-APIs we only have interfaces). So what's the problem if we have an REST-Endpoint like this: The upload fails because there's not way for REST-Easy to convert from the JSON-Encoded metaData to Java-Object (that's expected). Because of that we register "ParamConverterProvider" [1] for those types. This works great for @PathParam(), @QueryParam but fails for the @MultipartForm. From looking into the code I see no codepath leading the useage "ParamConverterProvider", hence my question is this by design or nobody yet came across that? I've prepared a simple test-case sample based on quarkus [2]. You can run that using "mvn clean test". Tom [1] https://github.com/tomsontom/rest-easy-mp/blob/master/src/main/java/at/be... [2] https://github.com/tomsontom/rest-easy-mp