Ok, reading all your pointers leaded nowhere, sadly. I could find a solution by browsing deeply in the Metronome issue tracker and documentation.
What I have worked is by setting it like this:
Host "*" interfaces = { "*" } ssl = { key = "/etc/yunohost/certs/upload.maillard.im/key.pem", certificate = "/etc/yunohost/certs/upload.maillard.im/crt.pem" } http_ports = {} https_ports = { 5290 }
Obviously, XMPP clients would refuse to upload without HTTPS and thus, yunohost should not mention it.