无法访问存储在PHP变量中的[object FormData]

I have called an AJAX with some post fields like this:

var postData ="gid="+gid+"&daydob=" +daydob+"&monthdob=" +monthdob;

With reference to THIS blog, for a FILE I have done something like this:

var fileId = document.getElementById("fileGS");                 
var file = fileId.files[0];                 
var formData = new FormData();
formData.append('file', file);

Finally:

postData = postData + "&fileGS=" + formData;
//Called AJAX after

Now while retrieving the parameter fileGS, I get [object FormData] in a PHP variable, where I am unable to access this JS object for uploading my File using PHP.

Can anyone please help me out that how can I access this object. I cannot alter much this AJAX since it is already in production(and uses YAHOO YUI). I am only adding image uploading feature to it.