clacks/lib/clacks_web/plug/format.ex

defmodule ClacksWeb.Plug.Format do
  import Plug.Conn

  def init(opts), do: opts

  def call(conn, _opts) do
    conn = fetch_query_params(conn)

    format =
      case conn.query_params do
        %{"format" => format} when format in ["activity+json", "html"] ->
          format

        %{"format" => "json"} ->
          "activity+json"

        _ ->
          Phoenix.Controller.get_format(conn)
      end

    assign(conn, :format, format)
  end
end