Added example program

MongoDB_GridFS_Helpers.cs

public static class Helpers
    {
        public static async Task<ObjectId> UploadFile(this GridFSBucket bucket, string localPath ,IProgress<BasicProgress> progress)
        {
            byte[] buffer = new byte[255000];
            
            ObjectId id;
            using (var localStream = File.OpenRead(localPath))
            using (var uploadStream = await bucket.OpenUploadStreamAsync(Path.GetFileName(localPath)))
            {
                id = uploadStream.Id;
                int read;
                while ((read = await localStream.ReadAsync(buffer, 0, buffer.Length)) > 0)
                {
                    if (progress != null)
                    {
                        progress.Report(new BasicProgress(localStream.Position, localStream.Length, id, localPath, ProgressType.Upload));
                    }
                    await uploadStream.WriteAsync(buffer, 0, read);
                    if (progress != null)
                    {
                        progress.Report(new BasicProgress(localStream.Position, localStream.Length, id, localPath, ProgressType.Upload));
                    }
                }

                if (progress != null)
                {
                    progress.Report(new BasicProgress(localStream.Position, localStream.Length, id, localPath, ProgressType.Complete));
                }
            }

            return id;
        }

        public static async Task DownloadFile(this GridFSBucket bucket, ObjectId id, string localPath, IProgress<BasicProgress> progress)
        {
            byte[] buffer = new byte[1024];

            Directory.CreateDirectory(Path.GetDirectoryName(localPath));
            using (var localStream = File.Create(localPath))
            using (var downloadStream = await bucket.OpenDownloadStreamAsync(id))
            {
                int read;
                while ((read = await downloadStream.ReadAsync(buffer, 0, buffer.Length)) > 0)
                {
                    await localStream.WriteAsync(buffer, 0, read);
                    if (progress != null)
                    {
                        progress.Report(new BasicProgress(localStream.Position, downloadStream.Length, id, localPath, ProgressType.Download));
                    }
                }

                if (progress != null)
                {
                    progress.Report(new BasicProgress(localStream.Position, localStream.Length, id, localPath, ProgressType.Complete));
                }
            }
        }
    }

    public enum ProgressType
    { 
        Upload,
        Download,
        Complete
    }
    public class BasicProgress
    {
        public ProgressType Type { get; }
        public FileInfo FileInfo { get; }
        public ObjectId Id { get; set; }
        public long TotalSize { get; }
        public long CurrentSize { get; }
        public double Percentage 
        {
            get => ((double)CurrentSize / (double)TotalSize) * 100;
        }

        public string Display { get => $"{RoundedPercentage(2)}%"; }


        public double RoundedPercentage(int decimalCount)
        {
            return Math.Round(Percentage, decimalCount);
        }

        public BasicProgress(long current, long total, ObjectId id, string path, ProgressType type)
        {
            this.CurrentSize = current;
            this.TotalSize = total;
            this.Id = id;
            this.FileInfo = new FileInfo(path);
            this.Type = type;

        }
    }

Program.cs

class Program
    {

        const string MongoDBConnectionString = "";
        
        static async Task Main(string[] args)
        {
            try
            {
                var progress = new Progress<BasicProgress>();
                progress.ProgressChanged += Progress_ProgressChanged;
                MongoClient dbClient = new MongoClient(MongoDBConnectionString);

                var database = dbClient.GetDatabase("Files");
                var bucket = new GridFSBucket(database);

                var file = @"D:\OneDrive\Documents\CIT368-01\Presentation_1080.mp4";
                
                Console.WriteLine("Uploading File");
                var id = await bucket.UploadFile(file, progress);
                Console.WriteLine("Downloading File");
                await bucket.DownloadFile(id, $@"D:\OneDrive\Desktop\Test{Path.GetExtension(file)}", progress);

                Console.WriteLine("Cleaning up");
                await bucket.DeleteAsync(id);
            }
            catch (Exception er)
            {

            }
        }

        private static void Progress_ProgressChanged(object sender, BasicProgress e)
        {
            switch (e.Type)
            {
                case ProgressType.Upload:
                    Console.Title = $"Uploading from {e.FileInfo.Name} to '{e.Id}' | {e.Display}";
                    break;
                case ProgressType.Download:
                    Console.Title = $"Downloading from '{e.Id}' to {e.FileInfo.FullName} | {e.Display}";
                    break;
                default:
                    break;
            }
        }
    }