fix export Bytes data type to JSON (#2409)

src/Adapters/Storage/Mongo/MongoTransform.js

@@ -781,6 +781,10 @@ const mongoObjectToParseObject = (className, mongoObject, schema) => {
             restObject[key] = GeoPointCoder.databaseToJSON(value);
             break;
           }
+          if (schema.fields[key] && schema.fields[key].type === 'Bytes' && BytesCoder.isValidDatabaseObject(value)) {
+            restObject[key] = BytesCoder.databaseToJSON(value);
+            break;
+          }
         }
         restObject[key] = nestedMongoObjectToNestedParseObject(mongoObject[key]);
       }
@@ -815,15 +819,29 @@ var DateCoder = {
 };
 
 var BytesCoder = {
+  base64Pattern: new RegExp("^(?:[A-Za-z0-9+/]{4})*(?:[A-Za-z0-9+/]{2}==|[A-Za-z0-9+/]{3}=)?$"),
+  isBase64Value(object) {
+    if (typeof object !== 'string') {
+      return false;
+    }
+    return this.base64Pattern.test(object);
+  },
+
   databaseToJSON(object) {
+    let value;
+    if (this.isBase64Value(object)) {
+      value = object;
+    } else {
+      value = object.buffer.toString('base64');
+    }
     return {
       __type: 'Bytes',
-      base64: object.buffer.toString('base64')
+      base64: value
     };
   },
 
-  isValidDatabaseObject(object) {
-    return (object instanceof mongodb.Binary);
+  isValidDatabaseObject(object) {    
+    return (object instanceof mongodb.Binary) || this.isBase64Value(object);
   },
 
   JSONToDatabase(json) {