или так
var bot_id = project.Variables["bot_id"].Value;
var chat_id = project.Variables["chat_id"].Value;
string File = project.Variables["File"].Value;
string data = "";
data += "--8d769bc78125cd4\n";
data += string.Format("Content-Disposition: form-data; name=\"document\"; filename=\"{0}\"\n", File);
data += "Content-Type: application/octet-stream\n";
data += File;
//var data = string.Format(@"--8d769bc78125cd4"+"\n"+@"Content-Disposition: form-data; name=""document""; filename=""3.png"""+"\n"+@"Content-Type: application/octet-stream"+"\n" + File, File);
string resRequest = ZennoPoster.HttpPost(
"
https://api.telegram.org/bot" + bot_id + "/sendDocument?chat_id=" + chat_id,
data,
"multipart/form-data",
"",
"UTF-8",
ZennoLab.InterfacesLibrary.Enums.Http.ResponceType.HeaderAndBody,
30000,
"",
"Mozilla/5.0 (Windows NT 10.0; WOW64; rv:45.0) Gecko/20100101 Firefox/45.0",
true,
5,
new[] {"Host: api.telegram.org"}
);