Read hidden field in server side

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ASP.NET

Hello everyone,
I set value to hiddenfield in client side and then i tried to read the value in server side. I can't read that value. The value came always empty string. Can u anyone help me.

Here my code :

function DownloadFile(control) {
                   $("#<%=hdnSelectedFile.ClientID%>").val(control.innerHTML);
                   alert("hidden field value : " + $("#<%=hdnSelectedFile.ClientID%>").val())
                   $("#<%=btnDownloadFile.ClientID%>").click();
               }

protected void btnDownloadFile_Click(object sender, EventArgs e)
   {
       try
       {
           // the file name to get
           string fileName = hdnPreviousUploadFileNames.Value;
           // get the file bytes to download to the browser
           string path = Server.MapPath(@"testpath\");
           byte[] fileBytes = System.IO.File.ReadAllBytes(path + fileName);
           // NOTE: You could also read the file bytes from a database as well.
           StreamFileToBrowser(fileName, fileBytes);
       }

       catch (Exception ex)
       {

       }
   }

Thanks for Advance..