function FormClick () {
  centerPopup();
  loadPopup();
  var str = $("#formadd").serialize();
  $.post("post.php", str, function(data) {
    disablePopup();
	alert('status: ' + data);
  });
}

<FORM action="post.php" method="post" enctype="multipart/form-data" name="formadd" id="formadd">
<INPUT type="file" name="file">    
<INPUT type="button" name="add" value="Add" onclick="FormClick(); return false">
</FORM>

<?php
 sleep (3);
 echo $_FILES['file']['name'];
?>