var x = 1;
var div = document.getElementById('files');

function add_field()
{  
  if (document.getElementById('file['+x+']').value.replace(/^([\s\n\r\t]+)$/, '') == '') { return false; }
  x++;
  ff = document.createElement('input');
  ff.type = 'file';
  ff.name = 'file['+x+']';
  ff.id = 'file['+x+']';
  ff.onchange = add_field;
  br = document.createElement('br');
  
  div.appendChild(ff);
  div.appendChild(br);
}