@callback http_callback(Plug.Conn.t(), Map.t()) ::
{:ok, Plug.Conn.t()}
| {:ok, Plug.Conn.t(), file_spec()}
| {:error, Plug.Conn.t(), String.t()}
@optional_callbacks http_callback: 2
@callback http_callback(Plug.Conn.t(), Map.t()) ::
{:ok, Plug.Conn.t()}
| {:ok, Plug.Conn.t(), file_spec()}
| {:error, Plug.Conn.t(), String.t()}
@optional_callbacks http_callback: 2