How to download a media item in sitecore from a page as “save as”

        private FileField _fileField;
        private Item _item;
        private MediaItem _mediaItem;

_fileField = Sitecore.Context.Item.Fields["File"];

            if (_fileField != null) 
                _item = Sitecore.Context.Item.Database.GetItem(_fileField.MediaID);

            if (_item != null)
                _mediaItem = new MediaItem(_item);

                    if (_mediaItem != null)
                    {

                   Button viewButton = new Button
                        {
                            CssClass = "ButtonCSSClass",
                            Text = "Click here to download the File",
                            ToolTip = "Click here to download the File",
                        };

                        viewButton.CommandArgument = _mediaItem.ID.ToString();
                        viewButton.Command += ViewButton;
                    }




protected void ViewButton_Click(object sender, CommandEventArgs e)
        {
            string mediaId = (String)e.CommandArgument;
            Button button = (Button)sender;

            if (mediaId.IsNullOrEmpty())               
                return;

            ID id = new ID(mediaId);

            if (id.IsNull)
                return;

            MediaItem mediaItem = Sitecore.Context.Database.Items.GetItem(id);

            if (mediaItem != null)
            {
                Stream stream = mediaItem.GetMediaStream();
                long fileSize = stream.Length;
                byte[] buffer = new byte[(int)fileSize];
                stream.Read(buffer, 0, (int)stream.Length);
                stream.Close();
                Response.Clear();
                Response.ContentType = String.Format(mediaItem.MimeType);
                Response.AddHeader("content-disposition", "attachment; filename=" + mediaItem.Name + "." + mediaItem.Extension);
                Response.Cache.SetCacheability(HttpCacheability.NoCache);
                Response.BinaryWrite(buffer);
                Response.End();
            }
        }