.NET Framework
Загрузка файлов и данных POST на веб-сервер

Загрузить файл с помощью WebRequest

Чтобы отправить файл и сформировать данные в одном запросе, содержимое должно иметь тип multipart / form-data .

using System;
using System.Collections.Generic;
using System.IO;
using System.Net;
using System.Threading.Tasks;

public async Task<string> UploadFile(string url, string filename, 
    Dictionary<string, object> postData)
{
    var request = WebRequest.CreateHttp(url);
    var boundary = $"{Guid.NewGuid():N}"; // boundary will separate each parameter
    request.ContentType = $"multipart/form-data; {nameof(boundary)}={boundary}";
    request.Method = "POST";

    using (var requestStream = request.GetRequestStream())
    using (var writer = new StreamWriter(requestStream))
    {
        foreach (var data in postData)
            await writer.WriteAsync( // put all POST data into request
                $"\r\n--{boundary}\r\nContent-Disposition: " +
                $"form-data; name=\"{data.Key}\"\r\n\r\n{data.Value}");

        await writer.WriteAsync( // file header
            $"\r\n--{boundary}\r\nContent-Disposition: " +
            $"form-data; name=\"File\"; filename=\"{Path.GetFileName(filename)}\"\r\n" +
            "Content-Type: application/octet-stream\r\n\r\n");

        await writer.FlushAsync();
        using (var fileStream = File.OpenRead(filename))
            await fileStream.CopyToAsync(requestStream);

        await writer.WriteAsync($"\r\n--{boundary}--\r\n");
    }

    using (var response = (HttpWebResponse) await request.GetResponseAsync())
    using (var responseStream = response.GetResponseStream())
    {
        if (responseStream == null)
            return string.Empty;
        using (var reader = new StreamReader(responseStream))
            return await reader.ReadToEndAsync();
    }
}

var response = await uploader.UploadFile("< YOUR URL >", "< PATH TO YOUR FILE >",
    new Dictionary<string, object>
    {
        {"Comment", "test"},
        {"Modified", DateTime.Now }
    });