How can I create custom error page or function of Plug.Parsers

Hello, I need to create a custom massage or page for Sending large request, for example if my user sends a file larger than 2 mg, my code shows them a page or redirect or something except default massage,

I have edited this line in endpoint.ex

  plug Plug.Parsers,
    parsers: [:urlencoded, :multipart, :json],
    pass: ["*/*"],
    body_reader: {WeddingCardWeb.DashboardController, :body_of_upload, []},
    json_decoder: Phoenix.json_library()

and my functions in my controller is:

  def body_of_upload(conn, _opts) do
    conn
    |> put_flash(:error, "متاسفانه فایل ارسالدر مواردی مثل حجم و فرمت مجاز دارای مشکل می باشد.")
    |> redirect(to: "/dashboard")
  end

but it doesn’t work.

By the way, can I create a preventer to limit my user sending upload file in my function:

  def upload(conn, params) do
      with {:ok, user_id} <- check_session(conn, :user_login_id), {:ok, invoice_id_valid} <- Ecto.UUID.cast(params["invoice_id"]),
           {:ok, user_infos} <- WeddingCardWeb.LoginController.user_find_pattern(user_id), true <- user_infos.status,
           {:ok, invoice_info}  <- invoice_pattern_by_id(invoice_id_valid),
           {:ok, :belong_user} <- invoice_belong_user_pattern(user_id, invoice_info.wedding_user_id),
           {:ok, upload} <- upload_check(params["upload"]), {:ok, [file_name]} <- file_name_reader(upload),
           true <- validate_file(file_name),
           nil <- WeddingCard.DB.UploadQuery.get_upload_by_invoice_id(invoice_info.id)  do

           [%{file_size: size, file_name: name_of_file}] = config_file(upload)
             # Store in db
             WeddingCard.DB.UploadQuery.insert_upload(%{
                 "wedding_user_id" => user_infos.id,
                 "wedding_invoice_id" => invoice_info.id,
                 "size" => "#{size}",
                 "address" => "#{name_of_file}"
               })
             conn
             |> put_flash(:info, "فایل شما با موفقیت آپلود گردید")
             |> redirect(to: "/dashboard")
      else
        _ ->
          conn
          |> put_flash(:error, "متاسفانه فایل مذکور ارسال نگردید یا در فرمت و حجم مناسب نبوده است. توجه فرمایید شما فقط مجاز به ارسال  فایل برای یک بار هستید.")
          |> redirect(to: "/dashboard")
      end
  end

all the functions I called top in my function, they work after uploaded file!!!

for example:

  def validate_file(filename) do
    ~w(.jpg .jpeg .xlsx .png .JPG .pdf .docx) |> Enum.member?(Path.extname(filename))
  end

it works after uploading, but the file concerned is bigger than my limitation, and my project shows me a default massage like this that I don’t want it.

Request Entity Too Large

Thanks