Between 2.7.5 and 2.7.6 there was a backward-incompatible change in behavior of reateasy-reactive. Now all endpoints, which produce multipart, should return org.jboss.resteasy.reactive.RestResponse and not javax.ws.rs.core.Response. This change is not in the upstream migration guide, but should be put into RHBQ Release Notes.

This change was discussed in comments to PR: https://github.com/quarkusio/quarkus/pull/24447/files#r888003036

gandrian can provide further details