Attach File to ListItem

public void AttachFileToListItem

(

string siteUrl,  

string webName, 

string listName,  

int itemId,  

string fileName, 

bool overwrite)
{
    using (ClientContext clientContext = new ClientContext(siteUrl))
    {
        FileStream fileStream = new FileStream(fileName, FileMode.Open);
        string attachmentPath = string.Format("/{0}/Lists/{1}/Attachments/{2}/{3}", 

webName, listName, itemId, Path.GetFileName(fileName));
        File.SaveBinaryDirect(clientContext, attachmentPath, fileStream, overwrite);
    }
}