Broken code (AddFile fails with byte[] around 50k or more)

pWebRequest.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Net;
using System.Text;
using System.Threading;
using Newtonsoft.Json;

namespace osu_common.Helpers
{
    public class pWebRequest
    {
        public string Url;
        internal bool Aborted { get; private set; }

        public event RequestUpdateHandler Updated;
        public event RequestCompleteHandler Finished;
        public event RequestStartedHandler Started;

        public Dictionary<string, object> Parameters = new Dictionary<string, object>();


        public pWebRequest(string url, params object[] args)
        {
            if (args.Length == 0)
                Url = url;
            else
                Url = string.Format(url, args);
        }

        private HttpWebRequest request;
        public HttpWebRequest Request
        {
            get
            {
                if (request == null) request = CreateWebRequest();
                return request;
            }
        }

        public WebResponse Response;
        Stream responseStream;

        protected virtual HttpWebRequest CreateWebRequest()
        {
            return WebRequest.Create(Url) as HttpWebRequest;
        }

        private int BytesRead;
        private int ContentLength;

        const int buffer_size = 8192;
        private byte[] buffer;

        private MemoryStream bodyStream = new MemoryStream();

        protected virtual Stream CreateOutputStream()
        {
            return new MemoryStream();
        }

        public Stream ResponseStream;

        public string ResponseString
        {
            get
            {
                try
                {
                    ResponseStream.Seek(0, SeekOrigin.Begin);
                    StreamReader r = new StreamReader(ResponseStream, Encoding.UTF8);
                    return r.ReadToEnd();
                }
                catch
                {
                    return null;
                }
            }
        }

        public byte[] ResponseData
        {
            get
            {
                try
                {
                    using (MemoryStream ms = new MemoryStream((int)ResponseStream.Length))
                    {
                        ResponseStream.Seek(0, SeekOrigin.Begin);
                        ResponseStream.CopyTo(ms);
                        return ms.ToArray();
                    }
                }
                catch
                {
                    return null;
                }
            }
        }

        public virtual void Perform()
        {
            ResponseStream = CreateOutputStream();

            Request.UserAgent = @"osu!";
            Request.AutomaticDecompression = DecompressionMethods.GZip | DecompressionMethods.Deflate;

            if (Parameters.Count > 0)
            {
                string boundary = @"-----------------------------28947758029299";

                Request.ContentType = string.Format(@"multipart/form-data; boundary={0}", boundary);

                foreach (KeyValuePair<string, object> p in Parameters)
                {
                    bodyStream.WriteLineExplicit("--" + boundary);

                    if (p.Value is byte[])
                    {
                        bodyStream.WriteLineExplicit("Content-Disposition: form-data; name=\"{0}\"; filename=\"{0}\"", p.Key);
                        bodyStream.WriteLineExplicit("Content-Type: application/octet-stream");
                        bodyStream.WriteLineExplicit();

                        byte[] data = (byte[])p.Value;
                        bodyStream.Write(data, 0, data.Length);

                        bodyStream.WriteLineExplicit();
                        bodyStream.WriteLineExplicit();
                    }
                    else
                    {
                        bodyStream.WriteLineExplicit("Content-Disposition: form-data; name=\"{0}\"", p.Key);

                        bodyStream.WriteLineExplicit();
                        bodyStream.WriteLineExplicit(p.Value);
                        bodyStream.WriteLineExplicit();
                    }
                }

                bodyStream.WriteLineExplicit("--" + boundary + "--");
                bodyStream.Flush();
            }

            if (bodyStream.Length > 0)
            {
                //POST
                Request.Method = @"POST";
                Request.ContentLength = bodyStream.Length;

                Request.BeginGetRequestStream(iar =>
                {
                    Stream requestStream = Request.EndGetRequestStream(iar);

                    bodyStream.Position = 0;
                    byte[] buff = new byte[buffer_size];
                    int read = 0;
                    while ((read = bodyStream.Read(buff, 0, buffer_size)) > 0)
                    {
                        requestStream.Write(buff, 0, read);
                        requestStream.Flush();
                        Console.WriteLine(read.ToString());
                    }


                    requestStream.Close();

                    beginResponse();
                }, null);
            }
            else
            {
                //GET
                beginResponse();
            }
        }

        private void beginResponse()
        {
            Request.BeginGetResponse(iar =>
            {
                try
                {
                    Response = Request.EndGetResponse(iar);
                    ContentLength = (int)Response.ContentLength;
                    responseStream = Response.GetResponseStream();

                    buffer = new byte[buffer_size];

                    responseStream.BeginRead(buffer, 0, buffer_size, readCallback, null);
                }
                catch (Exception e)
                {
                    Complete(e);
                }
            }, null);
        }

        private void readCallback(IAsyncResult result)
        {
            try
            {
                int readBytes = responseStream.EndRead(result);

                if (readBytes > 0)
                {
                    BytesRead += readBytes;
                    ResponseStream.Write(buffer, 0, readBytes);
                    responseStream.BeginRead(buffer, 0, buffer_size, readCallback, null);
                }
                else
                {
                    ResponseStream.Seek(0, SeekOrigin.Begin);
                    Complete();
                }
            }
            catch (Exception e)
            {
                Complete(e);
            }

        }

        public virtual void BlockingPerform(int timeout = 0)
        {
            Exception exc = null;
            bool completed = false;

            Finished += delegate (pWebRequest r, Exception e)
            {
                completed = true;
                exc = e;
            };

            Perform();

            int waitedTime = 0;
            while (!completed)
            {
                Thread.Sleep(20);
                waitedTime += 20;
                if (timeout > 0 && waitedTime > timeout)
                {
                    Abort();
                    throw new TimeoutException();
                }
            }

            if (exc != null) throw exc;
        }

        protected virtual void Complete(Exception e = null)
        {
            RequestCompleteHandler finished = Finished;
            if (finished != null)
                finished(this, e);
        }

        public virtual void Abort()
        {
            Aborted = true;
            if (request != null)
                request.Abort();
        }

        public delegate void RequestUpdateHandler(pWebRequest request, long current, long total);
        public delegate void RequestCompleteHandler(pWebRequest request, Exception e);
        public delegate void RequestStartedHandler(pWebRequest request);

        public void AddHeader(string key, string val)
        {
            Request.Headers.Add(key, val);
        }

        public void AddRaw(Stream stream)
        {
            stream.CopyTo(bodyStream);
        }

        public void AddRaw(byte[] data)
        {
            bodyStream.Write(data, 0, data.Length);
        }

        public void AddFile(string key, byte[] data)
        {
            Parameters.Add(key, data);
        }

        public void AddParameter(string key, string val)
        {
            Parameters.Add(key, val);
        }
    }

    public class pWebRequest<T> : pWebRequest
    {
        public pWebRequest(string url, params object[] args) : base(url, args)
        {
        }

        public T ResponseObject
        {
            get
            {
                return JsonConvert.DeserializeObject<T>(ResponseString);
            }
        }
    }

    /// <summary>
    /// Downloads a file from the internet to a specified location
    /// </summary>
    public class pFileWebRequest : pWebRequest
    {
        public string Filename;

        protected override Stream CreateOutputStream()
        {
            return new FileStream(Filename, FileMode.Create, FileAccess.Write, FileShare.Write, 32768);
        }

        public pFileWebRequest(string filename, string url) : base(url)
        {
            Filename = filename;
        }

        protected override void Complete(Exception e = null)
        {
            if (ResponseStream != null) ResponseStream.Close();
            if (e != null) GeneralHelper.FileDelete(Filename);
            base.Complete(e);
        }
    }

    public static class ExtensionMethods
    {
        public static void WriteLineExplicit(this Stream s)
        {
            s.WriteLineExplicit(string.Empty);
        }

        public static void WriteLineExplicit(this Stream s, object obj)
        {
            s.WriteLineExplicit(obj.ToString());
        }

        public static void WriteLineExplicit(this Stream s, string str, params object[] param)
        {
            if (param.Length > 0)
                str = string.Format(str, param);
            str += "\r\n";
            byte[] data = Encoding.ASCII.GetBytes(str);
            s.Write(data, 0, data.Length);
        }
    }
}

Line 128: The second end-line being written adds two bytes to the data when I try using it, which might be the problem. No idea how you parse this server side though so it might just work for you anyway. I was using http://www.posttestserver.com/ to test it.