In the settings of the uploader you can do this...

        multipart_params: {
            'key': 'path/on/s3/${filename}', // use filename as a key
            'Filename': '${filename}', // adding this to keep consistency across the runtimes
            'acl': 'public-read',
            'success_action_status': '201',
            'AWSAccessKeyId' : '<?php echo $access_key; ?>',
            'policy': '<?php echo $policy; ?>',
            'signature': '<?php echo $signature; ?>'
        },