It was my mistake. I have two ingress-paths in my cluster; one for internal services, one for external. My own relay runs on the same cluster as hanami and was only serving traffic to external requests while cluster-internal requests couldn't even resolve the hostname of my relay. Now it's all sorted out, uploads work fine, file management works great.

I'm gonna stick with hanami for my blossom needs. Once I've settled on a sane setup I'll PR my Kubernetes manifests.