Update prepareVCardParts function

javascript/src/utilities/emailUtilities.js

@@ -142,7 +142,7 @@ export const prepareVCardParts = parts => {
   }
   for (const part of parts) {
     if (part.body) {
-      if (part.body instanceof String) {
+      if (typeof part.body === "string") {
         part.body = stringToUtf8Base64(part.body);
       } else
       if (part.body instanceof Uint8Array) {