const
 URL_POST = 'http://localhost/gate/gate.php';

...

procedure POSTFile(Filename: String);
var
  IdHTTP: TIdHTTP;
  LPostStream: TIdMultiPartFormDataStream;
  LResultString: String;
  ff: TFileStream;

begin
  IdHTTP := TIdHTTP.Create(nil);
  IdHTTP.MaxLineLength := 0;
  LPostStream := TIdMultiPartFormDataStream.Create;
  ff := TFileStream.Create(Filename, 0);
  try
    LPostStream.AddObject('data', 'application/vnd', ff, Filename);
    try
      LResultString := IdHTTP.Post(URL_POST, LPostStream);
    except on E:Exception do
      Showmessage(E.Message);
    end;
    ShowMessage(LResultString);
  finally
    IdHTTP.Free;
    LPostStream.Free;
  end;
end;