How to register a post service to an Eliom client/server app?

Hi !
I encounter some difficulties trying to articulate well client and server code in eliom. I fail registering a Post service to an Eliom app and I do not figure why.
Here some code that I tried, as an example to start the discussion :

open%shared Eliom_content.Html.D
              
let%server service = Eliom_service.create
    ~path:(Eliom_service.Path ["upload"])
    ~meth:(Eliom_service.Get Eliom_parameter.unit)
    ()

let%client service = ~%service

let%shared name () = "demo-upload"

let%shared page_class = "dg-page-demo-upload"

let%server upload_service = Eliom_service.create
   ~path:(Eliom_service.Path ["upload"])
   ~meth:(Eliom_service.Post (Eliom_parameter.unit,
                              Eliom_parameter.file "file"))
   ()

let%client upload_service = ~%upload_service
    
let%shared () =
  Docgenius_base.App.register
    ~service:upload_service
    (fun () file ->
       let () = [%server ( some_code_that_does_smth_server_side : unit ) ] in
       Lwt.return
         (html
            (head (title (txt "Upload")) [])
            (body [h1 [txt "ok"]])))

let%shared page () =
  let f =
    (Form.post_form upload_service
       (fun file ->
          [p [Form.file_input ~name:file ();
              br ();
              Form.input ~input_type:`Submit ~value:"Send" Form.string
             ]]) ()) in
  Lwt.return
    [ h1 [txt "Upload"]
    ; p [txt "form upload"]
    ; f
    ]

Above code is included in ocsigen-start as a demo page, and leads to following error message :

File "demo_upload.eliom", line 28, characters 19-25:
Error: Uninterpreted extension 'server'.

I know from https://ocsigen.org/tuto/6.4/manual/misc that I can create a post service with Eliom_service.create and then register an action. This is fine, but I cannot figure out how to notify back easily that a server side computation were fine (the document were properly uploaded), or notify back a result. Maybe should I register an ocaml handler instead of an action for this purpose ?

Best !