let tryUploadFileAsync fileType =
  async {
    match fileType with
    | "image" ->
      return Some 1L |> Result.Ok // 1L is file id
    | "uploading_not_required" -> return None |> Result.Ok
    | _ -> return Result.Error (sprintf "%s uploading is not supported" fileType)
  }

let files = []
let uploadedFiles = System.Collections.Generic.List<int64>(files.Length)

for file in files do
  let! uploadResult = tryUploadFileAsync file
  match uploadResult with
  | Result.Ok (Some r) ->
    uploadedFiles.Add(r)
  | Result.Ok None ->
    uploadedFiles.Add(-1L)
  | Result.Error e -> return e |> Result.Error

m.UploadedFiles <- uploadedFiles

return serialize m |> Result.Ok