ReceiveFiles.cs

using System;
using System.IO;
using System.Linq;
using System.Threading.Tasks;
using RxTelegram.Bot.Api;
using RxTelegram.Bot.Interface.BaseTypes;
using RxTelegram.Bot.Interface.BaseTypes.Requests.Messages;
using File = System.IO.File;

namespace RxTelegram.Bot.Examples.ReceiveFiles;

internal static class ReceiveFiles
{
    private static TelegramBot Bot { get; set; }
    private const string BotToken = "<PASTE YOUR BOT TOKEN HERE>";

    public static async Task Main()
    {
        Bot = new TelegramBot($"{BotToken}");
        var me = await Bot.GetMe();
        Console.WriteLine($"Bot name: @{me.Username}");

        var subscription = Bot.Updates.Message.Subscribe(HandleReceivedMessage,
            exception => Console.WriteLine($"An error has occured: {exception.Message}"));
        Console.ReadLine();
        subscription.Dispose();
    }

    private static void HandleReceivedMessage(Message message)
    {
        RxTelegram.Bot.Interface.BaseTypes.File file = null;
        if (message.Photo != null && message.Photo.Any())
        {
            var photo = message.Photo.MinBy(x => x.Height);
            if (photo != null) file = Bot.GetFile(photo.FileId).Result;
        }
        else if (message.Document != null)
        {
            file = Bot.GetFile(message.Document.FileId).Result;
        }
        else if (message.Video != null)
        {
            file = Bot.GetFile(message.Video.FileId).Result;
        }
        else if (message.Audio != null)
        {
            file = Bot.GetFile(message.Audio.FileId).Result;
        }

        if (file == null)
        {
            Bot.SendMessage(new SendMessage { ChatId = message.Chat.Id, Text = "Please send me a file!" });
            return;
        }

        var photoData = Bot.DownloadFileByteArray(file).Result;
        var filename = file.FilePath.Split("/").LastOrDefault();
        File.WriteAllBytes(
            Path.Combine(Environment.CurrentDirectory, $"Output/{filename}"), photoData);
    }
}