s3: in v3, Body is a stream

https://docs.aws.amazon.com/sdk-for-javascript/v3/developer-guide/migrate-s3.html

src/storage/s3.js

@@ -199,10 +199,11 @@ class S3MultipartDownloadStream extends Readable {
         if (error) return this._handleError(error);
 
         const contentLength = parseInt(data.ContentLength, 10); // should be same as length
+        const body = await data.Body.transformToString();
 
         if (contentLength > 0) {
             this._readSize += contentLength;
-            this.push(data.Body);
+            this.push(body);
         } else {
             this._done();
         }