Need To Upload files with PHP and Ajax

["ITP Vision Promotion" <itpvision@xxxxxxxxx>]

Hello,
I have a registration page, it is processed to a php script through
ajax and javascript,
 The script worked until a input type file is involved

  var params = "username=" +
encodeURI(document.getElementById("usernamee").value)+
   "&password=" + encodeURI(document.getElementById("passwordd").value)+
   "&name=" + encodeURI(document.getElementById("name").value)+
   "&pic=" + pik +
   ;

As you may see,the  value retrived from the input type file with id
and name as pic, this will send a usual $_POST['pic'] , while I need
to Process _$FILE['pic'];
How can I send the value of a file
Many Thanks