ajaxcontroltoolkit setting hidden value after asyncfileupload has completed

Question

I have an asyncfileupload control that I'm using from the ajaxcontroltoolkit. On the file complete in the code behind I process the file and write the information in the file to a database. I get the id of the record from the database, and this needs to be written to an asp hidden field. I've tried just setting the value:

fldImageID.Value = pimg.IdImageGroup.ToString();

I've tried Registering a script like I've seen in an example on a website:

  ScriptManager.RegisterClientScriptBlock(
         ImageFileUploader, 
         ImageFileUploader.GetType(), 
         "script1",
       "alert('hi'); top.document.getElementById('" 
       + fldImageID.ClientID 
         + "').value='" 
       + pimg.IdImageGroup.ToString() 
       + "'; top.document.getElementById('" 
     + lblError.ClientID 
        + "').innerHTML = 'image uploaded'",
   true);

I've just tried embedding javascript in a response.Write call from the method I've set to process the uploaded file. Nothing I've done has worked so far. After I've done everything the hidden field still does not contain the required value.