Hmm, not sure.  Here's the method I use to write the content: 

protected void WriteResponse(object content, string contentType, bool asAttachment, string filename, params object[] filenameArgs)
{
    if (content != null)
    {
        filename = string.Format(filename, filenameArgs);

        bool isBinary = content.GetType().IsAssignableFrom(typeof(byte[]));

        Response.ClearHeaders();
        Response.ClearContent();

        Response.ContentType = contentType;
        string header = string.Format("{0}Filename={1}",
            asAttachment
                ? "attachment; "
                : string.Empty,
            filename);
        Response.AppendHeader("Content-Disposition", header);
        try
        {
            if (isBinary)
            {
                Response.AppendHeader("Content-Length", ((byte[])content).Length.ToString());
                Response.Buffer = true;
                Response.BinaryWrite((byte[])content);
            }
            else
            {
                Response.AppendHeader("Content-Length",
                    Encoding.Default.GetByteCount(content.ToString()).ToString());
                Response.Write(content);
            }
        }
        catch (Exception)
        {
            Response.ClearContent();
        }
        finally
        {
            Response.End();
        }
    }
}

Which  I call like so:
WriteResponse(content, "application/vnd.ms-excel", true, "Filename.xls");
where content is the output of exporter.GetExportGridContent.