NGINX inline lua to read size of file

  location /util {
    allow all;
    default_type 'text\plain';
    root /dev/null;

          location /util/display_filesize_bytes {
                set_by_lua $my_var '
                        function fsize(filename)
                          local file = io.open(filename, "r")
                          local size = file:seek("end")
                          return size
                        end
                        function find_filename(query_string)
                          value, garbage_and = string.match(query_string, "filename=(.+)(&?)")
                          return value
                        end
                        return fsize("/mnt/" .. find_filename(ngx.var.query_string))';

                return 200 $my_var;
          }
  }

Example: /util/display_filesize_bytes?filename=test.txt will return the size of test.txt. Will return 500: Internal Server Error if the file does not exist or query string does not exist