S3 allows the assignment of user defined metadata.
User defined metadata resides in its own "metadata" array and is called by the "getUserMetadata" function of the S3 java SDK 
When NIFI pulls the object and if there is user defined metadata it adds the metadata key / value as its literal key name.
This could lead to unintended NiFi attributes being replace like "filename"
A better approach is to prepend the S3 pulled metadata with s3 similar to how Fetchs3 prepends the other metadata of the object with s3.<metadata key>