In my php script that PUT files to S3 using AWK SDK for PHP, I had to add in the meta data, as shown below, which did the trick:
$response = $s3->create_object('bucketname', 'mountpoint/'.$filename, array(
'body' => $json_data,
'contentType' => 'application/json',
'acl' => AmazonS3::ACL_PUBLIC,
'meta' => array(
'mode' => '33188', // x-amz-meta-mode
)
));
The mode "33188" defined the permissions "rw-r--r--" instead of "---------" in S3 bucket (but reflected only in the EC2 mounted folder), which was later inherited by the EC2 mounted drive.
Hope this helps someone. Let me know!