Move inline images of work item description field to VSTS

Program.cs

using Microsoft.TeamFoundation.Client;
using Microsoft.TeamFoundation.WorkItemTracking.Client;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading.Tasks;

namespace MoveInlineImages
{
    class Program
    {
        static void Main(string[] args)
        {
            string oldTfsurl = @"todo";
            string vstsUrl = @"https://todo.visualstudio.com";
            string teamProject = "todo";

            Uri tfsUri = new Uri(vstsUrl);
            TfsTeamProjectCollection _tpc = new TfsTeamProjectCollection(tfsUri);

            WorkItemStore ws = _tpc.GetService(typeof(WorkItemStore)) as WorkItemStore;

            string wiql = "SELECT * FROM WorkItems WHERE [System.TeamProject] = '" + teamProject + "'";

            WorkItemCollection wiCollection = ws.Query(wiql);

            WebClient client = new WebClient();
            NetworkCredential nc = new NetworkCredential();
            nc.UserName = "todo";
            nc.Domain = "todo";
            nc.Password = "todo";
            client.Credentials = nc;

            foreach (WorkItem wi in wiCollection)
            {
                string regExSearchForImageUrl = "(?<=<img.*src=\")[^\"]*";
                MatchCollection matches = Regex.Matches(wi.Description, regExSearchForImageUrl);

                string regExSearchFileName = "(?<=FileName=)[^=]*";
                foreach (Match match in matches)
                {
                    if (match.Value.Contains(oldTfsurl))
                    {
                        //save image locally and upload as attachment
                        Match newFileNameMatch = Regex.Match(match.Value, regExSearchFileName);
                        if (newFileNameMatch.Success)
                        {
                            string fullImageFilePath = @"E:\temp\ImageFileDownloads\" + newFileNameMatch.Value;
                            client.DownloadFile(match.Value, fullImageFilePath);

                            int attachmentIndex = wi.Attachments.Add(new Attachment(fullImageFilePath));
                            wi.Save();
                            string attachmentGuid = wi.Attachments[attachmentIndex].FileGuid;

                            string newImageLink = String.Format("{0}/WorkItemTracking/v1.0/AttachFileHandler.ashx?FileNameGuid={1}&amp;FileName={2}", _tpc.Uri.ToString(), attachmentGuid, newFileNameMatch.Value);

                            wi.Description = wi.Description.Replace(match.Value, newImageLink);
                            wi.Attachments.RemoveAt(attachmentIndex);
                            wi.Save();
                        }
                        else
                        {
                            //no match
                            Console.WriteLine("no match here");
                        }
                    }
                }
            }
            client.Dispose();
        }
    }
}

How to check whether the image is already exist with FileNameGuid?
or Where it is stored?