var fileContent;
var openFile = function (event) {
  fileContent = undefined;
  var input = event.target;
  var reader = new FileReader();
  reader.readAsDataURL(input.files[0]);
  reader.onload = function () {
    var header = ";base64,";
    var fileData = reader.result;
    fileContent = fileData.substr(fileData.indexOf(header) + header.length);
  };
};

...
yield oEnvelopedData.propset_Content(fileContent);
var encMessage = yield oEnvelopedData.Encrypt(cadesplugin.CADESCOM_ENCODE_BASE64);