Plug setup

elixir101.ex

defmodule Elixir101 do
  def prep, do: Elixir101.Builder.all
  def start do
    {:ok, _} = Plug.Adapters.Cowboy.http Elixir101.Server, []
  end
end

server.ex

defmodule Elixir101.Server do
  use Plug.Builder

  plug Plug.Static, at: "/static", from: :elixir101
  plug :not_found

  def not_found(conn, _) do
    send_resp conn, 404, "not found"
  end
end

@skovsgaard think on at as a mask on the url, it will keep looking on priv/static

So, on this case the request should be http://localhost:4000/static/index.html and your file should be stored on priv/static/index.html

if you change to plug Plug.Static, at: "/", from: :elixir101 your request would be http://localhost:4000/index.html (file on same location)