AddAttachment to Sharepoint 2010 List item CSOM
Below is the code  to add an attachment to sharePoint 2010 list item through CSOM with WebReference, 
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Microsoft.SharePoint.Client;
using System.IO;
using System.Data;
using System.Net;
namespace CSOMFileUpload
{
    class Program
    {
        static void Main(string[] args)
        {
            const string listName = "Test";                   
            var credentials = new NetworkCredential("manjunath.soreddy", "MS", "dev");
            var clientContext = new ClientContext("https://spdev.com/TestSite/");
            clientContext.Credentials =
credentials; 
            var list = clientContext.Web.Lists.GetByTitle(listName);
            var itemCreateInfo = new ListItemCreationInformation();
            var newItem = list.AddItem(itemCreateInfo);
            newItem["Title"] = "Soreddy Client API";
            newItem.Update();
            clientContext.ExecuteQuery();
            clientContext.Load(newItem);
            clientContext.ExecuteQuery();           
            string path = @"c:\OutPutLog.txt";
//Here Uploadfile.inv is a Webreference which we have added to SharePoint 2010 Site from Visual Studio to add follow below steps
// Solution Explorer---> Add Service Reference ---> Advanced-> Add Web Reference --> URL
           var refin = new Uploadfile.inv.Lists();
           refin.Credentials = CredentialCache.DefaultCredentials;
           refin.Url = "https://spdev.com/TestSite/");
/_vti_bin/Lists.asmx";
           refin.AddAttachment(listName,
newItem["ID"].ToString(), Path.GetFileName(path),
                                     
System.IO.File.ReadAllBytes(path));
        }
    }
}


 



