Amazon Rekognition API - IndexFaces prompting an error for external image id - When 'externalImageId' has folder structure in it - amazon-s3

Question

I am trying to invoke IndexFaces API but getting an error :
*"exception":"com.amazonaws.services.rekognition.model.AmazonRekognitionException",
"message": "1 validation error detected: Value 'postman/postworld/postman_female.jpg' at 'externalImageId' failed to satisfy constraint: Member must satisfy regular expression pattern: [a-zA-Z0-9_.\\-:]+ (Service: AmazonRekognition; Status Code: 400; Error Code: ValidationException; Request ID: 3ac46c4d-3358-11e8-abd5-d5fb3ad03e33)",
"path": "/enrolluser"*
I was able to upload my file successfully into S3 using the so called "folder structure"of S3 . But when I am trying to read the same file for IndexFaces , then it's prompting an error related to éxternalImageId'.
Here is the snapshot from the S3 of my uploaded file :
http://xxxxxx.s3.amazonaws.com/postman/postworld/postman_automated.jpg
If I get rid of folder structure and directly dump the file , like :
http://xxxxxx.s3.amazonaws.com/postman_automated.jpg
then the IndexFaces API is passing it successfully .
Can you please suggest how to pass the externalImageId when I do have the 'folder structure'? Currently I am passing the externalImageId through my java code like :
enrolledFileName = userName +"/"+myWorldName+"/"+enrolledFileName;
System.out.println("The FILE NAME MANIPULATED IS:"+enrolledFileName);
String generateAmazonFaceId = amazonRekognitionManagerObj.addToCollectionForEnrollment(collectionName, bucketName, enrolledFileName);
System.out.println("The Generated FaceId is:"+generateAmazonFaceId);**strong text**
Above code internally calls :
Image image=new Image().withS3Object(new S3Object().withBucket(bucketName)
.withName(fileName));
IndexFacesRequest indexFacesRequest = new IndexFacesRequest()
.withImage(image)
.withCollectionId(collectionName)
.withExternalImageId(fileName)
.withDetectionAttributes("ALL");