Attach file to post using C# RestSharp and Drupal Services
I have figured it out this way, sorry for late posting the answer:

// FIRST LOGIN
var client = new RestClient("http://....../");
var request = new RestRequest("user/login.json", Method.POST);
request.AddHeader("Content-Type", "x-www-form-urlencoded");
request.AddParameter("username", username);
request.AddParameter("password", password);
var restResponse = client.Execute&lt;Login&gt;(request);


Now upload a picture:

public string UploadPic(string path, Login login)
{
    var request = new RestRequest("file.json", Method.POST);
    request.RequestFormat = RestSharp.DataFormat.Json;
    request.AddCookie(login.session_name, login.sessid);
    request.AddHeader("X-CSRF-Token", login.token);
    request.AddParameter("uid", login.user.uid);
    string filevalue = System.Convert.ToBase64String(file_get_byte_contents(path));
    request.AddParameter("file", filevalue);
    request.AddParameter("filename", path.Split(new Char[] { '\\', '/' }).Last());
    var response = client.Execute&lt;DFile&gt;(request);

    return response.Data.fid;
}

static byte[] file_get_byte_contents(string fileName)
{
    byte[] sContents;
    if (fileName.ToLower().IndexOf("http:") &gt; -1)
    {
        // URL 
        System.Net.WebClient wc = new System.Net.WebClient();
        sContents = wc.DownloadData(fileName);
    }
    else
    {
        // Get file size
        FileInfo fi = new FileInfo(fileName);

        // Disk
        FileStream fs = new FileStream(fileName, FileMode.Open, FileAccess.Read);
        BinaryReader br = new BinaryReader(fs);
        sContents = br.ReadBytes((int)fi.Length);
        br.Close();
        fs.Close();
    }

    return sContents;
}


Where the class Login and DFile are the following: 

public class DFile
{
    public string fid { get; set; }
    public string uid { get; set; }
    public string filename { get; set; }
    public string uri { get; set; }
    public string filemime { get; set; }
    public string filesize { get; set; }
    public string status { get; set; }
    public string timestamp { get; set; }
}
public class Login
{
    public string sessid { get; set; }
    public string session_name { get; set; }
    public string token { get; set; }
    public User user { get; set; }
}