Need To Upload files with PHP and Ajax

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 Thank