No querystring neither file content for saving back a doc

Hi to all,
I have a problem saving a document when I call Writer's remote API.
First of all I make this request:

https://exportwriter.zoho.com/remotedoc.im
?url=[my web site]/tmp1.doc
&apikey=[my api key]
&output=url
&mode=normaledit
&filename=tmp1.doc
&lang=it
&id=23234
&format=doc
&saveurl=[my web site]/Save.aspx

Then I use output url in a frame and Zoho Writer Editor appears loading correctly my file (tmp1.doc).
After that, I edit my file adding some text and I click on Zoho Writer's button save.
My page Save.aspx get the response of Zoho request and it's ok.
According to this sample, code inside Save.aspx is this one:

protected void Page_Load(object sender, EventArgs e)
{
System.IO.StreamWriter writer;
string logMessage = "";
HttpPostedFile returnFile = null;
string filename = "";
string percorso = System.Web.HttpContext.Current.Request.MapPath(System.Web.HttpContext.Current.Request.ApplicationPath);
try
{
returnFile = Request.Files["content"];
filename = Request.QueryString["filename"];
Request.Files["content"].SaveAs(percorso + filename);
}
catch
{
}
Label1.Text = Request.QueryString.Count;
Label2.Text = Request.Files.Count;
}

My problem is that Questring is empty as the collection of files in the request.
I've made something wrong, could someone help me to understand what this problem is due to?
Thanks in advance,
Roby

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