I think you might want to read this thread : Images upload with XMPP / Metronome and http_upload: File upload with Metronome · Issue #1278 · YunoHost/issues · GitHub (bottom line is : currently it’s probably broken and help is welcome to provide a fix)
I think currently the status quo is “since metronome is now maintained again. the cost vs. interest of migrating to prosody is not so interesting for now…”