Generating XLSX file

I’m trying to dynamically generate a XLSX file, using elixlsx and I’m doing it, but the file generated doesn’t have any file extension, i.e. I just download a file called download and I’d like to download download.xlsx. Can someone help me. please?

My current relevant code is:

config/config.exs

# Configures xlsx mime type
config :mime, :types, %{
  "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet" => ["xlsx"]
 }

web/router.ex

defmodule Backoffice.Router do
  use Backoffice.Web, :router

  pipeline :browser do
    plug :accepts, ["html", "xlsx"]
    plug :fetch_session
    plug :fetch_flash
    plug :protect_from_forgery
    plug :put_secure_browser_headers
  end

  pipeline :api do
    plug :accepts, ["json"]
  end

  scope "/", Backoffice do
    pipe_through :browser # Use the default browser stack

    get "/",         PageController, :index  
    get "/download_form", PageController, :download_form 
    get "/download",   PageController, :download
  end
end

web/controllers/page_controller.ex

defmodule Backoffice.PageController do
  use Backoffice.Web, :controller

  def index(conn, _params) do
    render conn, "index.html"
  end

  # render download form
  def download_form(conn, _params) do
    render conn, "download_form.html"
  end

  # generates XLSX
  def download(conn, _params) do
    render conn, "download.xlsx"
  end
end

web/views/page_view.ex

defmodule Backoffice.PageView do
  use Backoffice.Web, :view

  import Elixlsx
  alias Elixlsx.Workbook
  alias Elixlsx.Sheet

  def render("download.xlsx", _params) do #, %{tweets: tweets}) do
    Workbook.append_sheet(%Workbook{}, Sheet.with_name("Sheet 1")
    |> Sheet.set_cell("A1", "Hello", bold: true))
    |> Elixlsx.write_to_memory("download.xlsx") |> elem(1)  |> elem(1)
  end
end