檔案選擇:

private void File_Click(object sender, RoutedEventArgs e)
{
    var openFileDialog = new Microsoft.Win32.OpenFileDialog();
    var result = openFileDialog.ShowDialog();
    if (result == true)
    {
        //將檔案路徑存入 Textbox 或 Label
        FILENAME.Content = openFileDialog.FileName;
    }
}

2019/01/10 更新程式
FormData 新增 ProgressChanged 事件，可以取得 百分比、上傳位元數、上傳項目數 等進度資訊。

用法:

try
{
    //邊界
    var boundary = "-----" + DateTime.Now.Ticks.ToString("x");

    var request = (HttpWebRequest)HttpWebRequest.Create("http://127.0.0.1/WebformPostTest_Web/Test.aspx");
    request.Method = "POST";
    request.ContentType = "multipart/form-data; boundary=" + boundary;

    //測試時設定 Proxy 到 Fiddler 上
    //request.Proxy = new WebProxy("127.0.0.1", 8888);

    var formData = new FormData(boundary, Encoding.UTF8);
    formData.Append(new FieldItem("USER", USER.Text));
    formData.Append(new FieldItem("SHOP", SHOP.Text));
    formData.Append(new FieldItem("TERM", TERM.Text));
    formData.Append(new FieldItem("FILETYPE", FILETYPE.Text));
    formData.Append(new FieldItem("SALES_DATE", SALES_DATE.Text));
    formData.Append(new FieldItem("CLOSE_COUNTER", CLOSE_COUNTER.Text));
    formData.Append(new FieldItem("BATCHID", BATCHID.Text));
    formData.Append(new FileItem("FILENAME",
        System.IO.Path.GetFileName(FILENAME.Content.ToString()),
        FILENAME.Content.ToString()));
    formData.Append(new FileItem("FILENAME",
        System.IO.Path.GetFileName(FILENAME2.Content.ToString()),
        FILENAME2.Content.ToString()));

    request.ContentLength = formData.GetLength();

    //開新執行緒上傳，才不會卡住UI
    await Task.Run(() =>
    {
        //進度處理
        formData.ProgressChanged += (s, ee) =>
        {
            //呼叫主執行緒更新UI
            this.Dispatcher.Invoke(() =>
            {
                //只想計算檔案的話，將 PostFiles 扣掉欄位數就等於檔案數
                var postFile = ee.PostItems - 7 > 0 ? ee.PostItems - 7 : 0;
                var totalFile = ee.TotalItems - 7 > 0 ? ee.TotalItems - 7 : 0;
                Process.Content =
                    $"百分比: {ee.Percentage}，上傳位元數: {ee.PostBytes}/{ee.TotalBytes}，上傳檔案數: {postFile}/{totalFile}";
            });
        };

        var requestStream = request.GetRequestStream();
        formData.Write(requestStream);
        requestStream.Close();

        var response = (HttpWebResponse)request.GetResponse();
    });
}
catch(Exception ex)
{
}

Multipart.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Multipart
{
    /// <summary>
    /// 表單
    /// </summary>
    public class FormData
    {
        /// <summary>
        /// 邊界
        /// </summary>
        public string Boundary { get; set; }

        /// <summary>
        /// 表單編碼
        /// </summary>
        public Encoding Encoding { get; set; }

        public FormData(string boundary, Encoding encoding)
        {
            Boundary = boundary;
            Encoding = encoding;
        }

        /// <summary>
        /// 緩衝區大小，預設100KB
        /// </summary>
        public int BufferSize { get; set; } = 102400;

        /// <summary>
        /// 所有表單項目
        /// </summary>
        private List<FormDataItem> itemList { get; set; } = new List<FormDataItem>();

        /// <summary>
        /// 將項目加入表單
        /// </summary>
        /// <param name="item"></param>
        public void Append(FormDataItem item)
        {
            itemList.Add(item);
        }

        /// <summary>
        /// 取得表單長度
        /// </summary>
        /// <returns></returns>
        public long GetLength()
        {
            var length = (long)0;
            foreach (var item in itemList)
            {
                length += item.GetLength(Boundary, Encoding);
            }
            //結尾
            length += Encoding.UTF8.GetBytes("--" + Boundary + "--\r\n").Length;
            return length;
        }

        /// <summary>
        /// 將表單寫入串流
        /// </summary>
        /// <param name="stream"></param>
        public void Write(Stream stream)
        {
            var postBytes = (long)0;
            var totalBytes = this.GetLength();
            var postItems = 0;
            var totalItems = itemList.Count;

            //觸發進度事件
            var progressFunc = (Action)(() =>
            {
                var process = (int)Math.Ceiling(((double)(postBytes)) / totalBytes * 100);

                OnProgress(postBytes, totalBytes, postItems, totalItems, process);
            });

            foreach (var item in itemList)
            {
                item.ProgressChanged += (s, e) =>
                {
                    postBytes += e.PostBytes;
                    progressFunc();
                };
                item.Write(stream, Boundary, Encoding, BufferSize);
                postItems++;
            }
            //結尾
            var bytes = Encoding.UTF8.GetBytes("--" + Boundary + "--\r\n");
            stream.Write(bytes, 0, bytes.Length);
            
            postBytes += bytes.Length;
            progressFunc();
        }

        /// <summary>
        /// 上傳進度事件
        /// </summary>
        public event FormDataProgressChangedEventHandler ProgressChanged;

        protected virtual void OnProgress(long postBytes, long totalBytes, int postItems, int totalItems, int percentage)
        {
            ProgressChanged?.Invoke(this, new FormDataProgressChangedEventArgs(postBytes, totalBytes, postItems, totalItems, percentage));
        }
    }

    /// <summary>
    /// 表單項目
    /// </summary>
    public abstract class FormDataItem
    {
        /// <summary>
        /// 取得項目位元組長度
        /// </summary>
        /// <param name="boundary"></param>
        /// <param name="encoding"></param>
        /// <returns></returns>
        internal abstract long GetLength(string boundary, Encoding encoding);

        /// <summary>
        /// 將項目寫入串流
        /// </summary>
        /// <param name="stream"></param>
        /// <param name="boundary"></param>
        /// <param name="encoding"></param>
        /// <param name="bufferSize"></param>
        internal abstract void Write(Stream stream, string boundary, Encoding encoding, int bufferSize);

        /// <summary>
        /// 上傳進度事件
        /// </summary>
        internal event FormDataItemProgressChangedEventHandler ProgressChanged;

        protected virtual void OnProgress(long postBytes)
        {
            ProgressChanged?.Invoke(this, new FormDataItemProgressChangedEventArgs(postBytes));
        }
    }

    /// <summary>
    /// 一般欄位
    /// </summary>
    public class FieldItem : FormDataItem
    {
        /// <summary>
        /// 樣板
        /// </summary>
        private static readonly string template;

        static FieldItem()
        {
            var temp = "";
            temp += "--{0}\r\n";
            temp += "Content-Disposition: form-data; name=\"{1}\"\r\n";
            temp += "\r\n";
            temp += "{2}\r\n";
            template = temp;
        }

        /// <summary>
        /// 表單的Key
        /// </summary>
        private string Key { get; set; }

        /// <summary>
        /// 欄位的值
        /// </summary>
        private string Value { get; set; }

        /// <summary>
        /// 一般欄位
        /// </summary>
        /// <param name="key">表單的Key</param>
        /// <param name="value">欄位的值</param>
        public FieldItem(string key, string value)
        {
            Key = key;
            Value = value;
        }

        /// <summary>
        /// Format 暫存
        /// </summary>
        private byte[] cache = null;
        private byte[] Format(string boundary, Encoding encoding)
        {
            if (cache == null)
            {
                cache = encoding.GetBytes(string.Format(template, boundary, Key, Value));
            }
            return cache;
        }

        internal override long GetLength(string boundary, Encoding encoding)
        {
            return this.Format(boundary, encoding).Length;
        }

        internal override void Write(Stream stream, string boundary, Encoding encoding, int bufferSize)
        {
            //寫入 template
            var bytes = this.Format(boundary, encoding);
            stream.Write(bytes, 0, bytes.Length);

            //觸發進度事件
            OnProgress(bytes.Length);
        }
    }

    /// <summary>
    /// 檔案
    /// </summary>
    public class FileItem : FormDataItem
    {
        /// <summary>
        /// 樣板
        /// </summary>
        private static readonly string template;

        static FileItem()
        {
            var temp = "";
            temp += "--{0}\r\n";
            temp += "Content-Disposition: form-data; name=\"{1}\"; filename=\"{2}\"\r\n";
            temp += "Content-Type: application/octet-stream\r\n\r\n";
            template = temp;
        }

        /// <summary>
        /// 表單的Key
        /// </summary>
        private string Key { get; set; }

        /// <summary>
        /// 檔案名稱
        /// </summary>
        private string FileName { get; set; }

        /// <summary>
        /// 檔案路徑
        /// </summary>
        private string FilePath { get; set; }

        /// <summary>
        /// 檔案
        /// </summary>
        /// <param name="key">表單的Key</param>
        /// <param name="fileName">檔案名稱</param>
        /// <param name="filePath">檔案路徑</param>
        public FileItem(string key, string fileName, string filePath)
        {
            Key = key;
            FileName = fileName;
            FilePath = filePath;
        }

        /// <summary>
        /// Format 暫存
        /// </summary>
        private byte[] cache = null;
        private byte[] Format(string boundary, Encoding encoding)
        {
            if (cache == null)
            {
                cache = encoding.GetBytes(string.Format(template, boundary, Key, FileName, FilePath));
            }
            return cache;
        }

        internal override long GetLength(string boundary, Encoding encoding)
        {
            return this.Format(boundary, encoding).Length +
                new FileInfo(FilePath).Length + encoding.GetBytes("\r\n").Length;
        }

        internal override void Write(Stream stream, string boundary, Encoding encoding, int bufferSize)
        {
            //寫入 template
            var bytes = this.Format(boundary, encoding);
            stream.Write(bytes, 0, bytes.Length);

            //觸發進度事件
            OnProgress(bytes.Length);

            //寫入檔案
            var buffer = new byte[bufferSize];
            using (var fs = new FileStream(FilePath, FileMode.Open, FileAccess.Read))
            {
                fs.Seek(0, SeekOrigin.Begin);

                var readCount = 0;
                while (true)
                {
                    readCount = fs.Read(buffer, 0, bufferSize);
                    if (readCount == 0)
                        break;
                    stream.Write(buffer, 0, readCount);

                    //觸發進度事件
                    OnProgress(readCount);
                }
            }

            //寫入最後的換行符號
            bytes = encoding.GetBytes("\r\n");
            stream.Write(bytes, 0, bytes.Length);

            //觸發進度事件
            OnProgress(bytes.Length);
        }
    }

    /// <summary>
    /// 處理 FormData ProgressChanged 事件的方法
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="e"></param>
    public delegate void FormDataProgressChangedEventHandler(object sender, FormDataProgressChangedEventArgs e);

    /// <summary>
    /// FormData ProgressChanged 事件的資料
    /// </summary>
    public class FormDataProgressChangedEventArgs : EventArgs
    {
        public FormDataProgressChangedEventArgs(long postBytes, long totalBytes, int postItems, int totalItems, int percentage)
        {
            PostBytes = postBytes;
            TotalBytes = totalBytes;
            PostItems = postItems;
            TotalItems = totalItems;
            Percentage = percentage;
        }

        /// <summary>
        /// 已上傳位元數量
        /// </summary>
        public long PostBytes { get; }

        /// <summary>
        /// 全部位元數量
        /// </summary>
        public long TotalBytes { get; }

        /// <summary>
        /// 已上傳項目數量
        /// </summary>
        public int PostItems { get; set; }

        /// <summary>
        /// 全部項目數量
        /// </summary>
        public int TotalItems { get; set; }

        /// <summary>
        /// 整體百分比
        /// </summary>
        public int Percentage { get; }
    }

    /// <summary>
    /// 處理 FormDataItem ProgressChanged 事件的方法
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="e"></param>
    internal delegate void FormDataItemProgressChangedEventHandler(object sender, FormDataItemProgressChangedEventArgs e);

    /// <summary>
    /// FormDataItem ProgressChanged 事件的資料
    /// </summary>
    internal class FormDataItemProgressChangedEventArgs : EventArgs
    {
        public FormDataItemProgressChangedEventArgs(long postBytes)
        {
            PostBytes = postBytes;
        }

        /// <summary>
        /// 送出的位元數量
        /// </summary>
        public long PostBytes { get; }
    }
}

這網頁我也有查到，但老實說我對他的內容不是很了解...(程式苦手)
是否大神您可以給小弟一個範例參考?

HttpRequest<-我試過但做不出來,就不知道哪裡有問題.是我程式碼哪兒有少嗎?
還是我上面打的方向跟本錯誤啊TAT

剛查了一下,若不用HttpRequest,改用WebClient的話也能達成同樣的結果嗎?

可以唷～

目前他是有一台server在接收這些資料，所以應該不用自己寫。
只是我實在搞不懂為啥他網頁寫的傳的到伺服器，但用C#卻實作不出來。
我卡在這裡。

對 您說的『他是有一台server在接收這些資料』<==資料庫嗎?
所以您要找出，他怎樣去將資料，傳給這一台server

要嗎他跟您說，怎要寫用javascript 、 jquery、EasyUI ...等等方式，
要嗎他要您自己寫DAO、JDBC ...等connection 方式

原網頁原始碼應該是ＡＳＰ。
ＳＵＢＭＩＴ是我在用Ｃ＃的ＷＩＮＦＲＯＭ在實作時建立出的按鍵。
我可以幫你翻譯！

WinForm, 所以你到底是要用WinForm還是要用WebForm ?

我大概知道你想做什麼了,
如果你是要POST資料過去,
你可以查一下 C# 爬蟲,
至於上傳檔案我沒有這樣做過,
或許可以試著用MVC的HttpPostedFileBase.

原來有現成的類別可以用~