Remote Sheet API encoding problem

Remote Sheet API encoding problem

Hi!
I´m trying to post a sheet through the remote api and I´m having some inconveniences with encoding.
Currently I send my files like this and works...
 
c#
 
            StringBuilder contents = new StringBuilder();
            //boundary
            contents.AppendLine(header);
            contents.AppendLine(string.Format("Content-Disposition: form-data; name=\"content\"; filename=\"{0}\"", fileName));
            contents.AppendLine("Content-Type: application/vnd.ms-excel");
            contents.AppendLine();
            contents.AppendLine(Encoding.Default.GetString(file));
            //boundary
            contents.AppendLine(header);
            //saveurl
            contents.AppendLine("Content-Disposition: form-data; name=\"saveurl\"");
            contents.AppendLine();
            contents.AppendLine("











http://www.some.com ");
            //boundary
            contents.AppendLine(header);
            //id           
            contents.AppendLine("Content-Disposition: form-data; name=\"id\"");
            contents.AppendLine();
            contents.AppendLine("adfwedf");
            //boundary
            contents.AppendLine(header);
            //format
            contents.AppendLine("Content-Disposition: form-data; name=\"format\"");
            contents.AppendLine();
            contents.AppendLine("xls");
            //footer
            contents.AppendLine(footer);













            byte[] bytes = Encoding.Default.GetBytes(contents.ToString());
            request.ContentLength = bytes.Length;           
            Stream requestStream = request.GetRequestStream();
            requestStream.Write(bytes, 0, bytes.Length);
            requestStream.Flush();
            requestStream.Close();


            WebResponse response = request.GetResponse();
            StreamReader reader = new StreamReader(response.GetResponseStream());
            string responseBody = reader.ReadToEnd();
            reader.Close();




However, If I use UTF8 (byte[] bytes = Encoding.UTF8.GetBytes) instead of the deafult encoding I receive a "500 internal server error".
Any thoughts?

Thanks in advance,

Gonzalo.