Exemplo de cliente enviando um arquivo via HTTP via multipart e recebendo outro de volta

PostAndGetFile.cs

public void Teste()
{
    var retorno = PostAndGetArquivo();

    byte[] bytesImagem;

    using (MemoryStream ms = new MemoryStream())
    {
        retorno.CopyTo(ms);
        bytesImagem = ms.ToArray();
    }

    File.WriteAllBytes($"c:\\teste.pdf", bytesImagem);
}

public Stream PostAndGetArquivo()
{
    var http = (HttpWebRequest) WebRequest.Create("http://xxx/yyy");
    http.Method = "POST";

    var sendFile = File.ReadAllBytes("c:\\exemplo.rtf");

    var byteArrayContent = new ByteArrayContent(sendFile);
    var multipartContent = new MultipartFormDataContent();

    multipartContent.Add(byteArrayContent, "file", "exemplo.rtf");

    foreach (KeyValuePair<string, IEnumerable<string>> multipartContentHeader in multipartContent.Headers)
    {
        http.Headers.Add(multipartContentHeader.Key,
            multipartContentHeader.Value.Aggregate((current, next) => $"{current},{next}"));
    }

    var stringToSend = new StreamReader(multipartContent.ReadAsStreamAsync().Result).ReadToEnd();
    
    using (var streamWriter = new StreamWriter(http.GetRequestStream()))
    {
        streamWriter.Write(stringToSend);
    }
   
    var response = http.GetResponse();

    return response.GetResponseStream();
}