set_body_file not working as expected

nginx.conf

  server {
    server_name  $hostname;
    listen 80;
    # add_header Content-Type application/json
    location /foo {
      proxy_pass http://127.0.0.1:2000;
    }

    location  / {
      client_body_in_file_only on;
      lua_need_request_body on;
      content_by_lua "
        ngx.req.set_body_file = '/tmp/nginx.conf'
        res = ngx.location.capture(
          '/foo/bar',
          { method = ngx.HTTP_PUT})
          ngx.say(res.body)
    ";
    }