save and retrive files from sql server

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C#

// Read the file and convert it to Byte Array
        string filePath = Server.MapPath("APP_DATA/TestDoc.doc");
        string filename = Path.GetFileName(filePath);

        FileStream fs = new FileStream(filePath, FileMode.Open, FileAccess.Read);
        BinaryReader br = new BinaryReader(fs);
        Byte[] bytes = br.ReadBytes((Int32)fs.Length);
        br.Close();
        fs.Close();

        //insert the file into database
        string strQuery = "insert into tblFiles(Name, ContentType, Data) values (@Name, @ContentType, @Data)";
        SqlCommand cmd = new SqlCommand(strQuery);
        cmd.Parameters.Add("@Name", SqlDbType.VarChar).Value = filename;
        cmd.Parameters.Add("@ContentType", SqlDbType.VarChar).Value = "application/vnd.ms-word";
        cmd.Parameters.Add("@Data", SqlDbType.Binary).Value = bytes;
        InsertUpdateData(cmd);

protected void Retreive_Doc(object sender, EventArgs e)
    {
        string strQuery = "select Name, ContentType, Data from tblFiles where id=@id";
        SqlCommand cmd = new SqlCommand(strQuery);
        cmd.Parameters.Add("@id", SqlDbType.Int).Value = 1;
        DataTable dt = GetData(cmd);
        if (dt != null)
        {
            download(dt);
        }
    }


private void download (DataTable dt)
    {
        //String[] strings=( String[])dt.Rows[0]["Data"];
        Byte[] bytes = (Byte[])dt.Rows[0]["Data"];
        Response.Buffer = true;
        Response.Charset = "";
        Response.Cache.SetCacheability(HttpCacheability.NoCache);
        Response.ContentType = dt.Rows[0]["ContentType"].ToString();
        Response.AddHeader("content-disposition", "attachment;filename="
        + dt.Rows[0]["Name"].ToString());
        Response.BinaryWrite(bytes);
        Response.Flush(); 
        Response.End();
    }

i get this error

Unable to cast object of type 'System.String' to type 'System.Byte[]'.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.InvalidCastException: Unable to cast object of type 'System.String' to type 'System.Byte[]'.

it insert data to database and dosen't retrive it