ElasticTranscoder でのサムネイルの拡張子変更方法

下記のサンプルソースで実装したのですがサムネイル作成時PNGで作成されます。

PNGがElasticTranscoderの仕様なのでしょうか？

それともどこかで変更可能なのでしょうか？

よろしくお願いします。

'use strict';

var AWS = require('aws-sdk');
var s3 = new AWS.S3({
    apiVersion: '2012-09-25'
});

var transcoder = new AWS.ElasticTranscoder({
    apiVersion: '2012-09-25',
    region: 'ap-northeast-1'
});

// return dirname without extensionn
function dirname(path) {
    var p = path.split(path.sep).pop().split('.')[0];
    return decodeURIComponent(p);
}

exports.handler = function(event, context) {

    console.log('Executing Elastic Transcoder Orchestrator');

    var bucket = event.Records[0].s3.bucket.name;
    if (bucket !== 'transcoder.raw') {
        context.fail('Incorrect Video Input Bucket');
        return;
    }
    
    var pipelineId = '<ElasticTranscoder Pipeline ID>';
    var key = event.Records[0].s3.object.key;
    var dkey = dirname(key);

    console.log("(^-^)key");    
    console.log(key);
    console.log(dkey);

    var params = {
        Input: {
          Key: key,
          FrameRate: 'auto',
          Resolution: 'auto',
          AspectRatio: 'auto',
          Interlaced: 'auto',
          Container: 'auto',
        },
        PipelineId: pipelineId,
        Outputs: [
          {
            Key: dkey + '/600k/s',
            PresetId: '1351620000001-200040', // hls 600k
            SegmentDuration: '10'
          }
          ,{
            Key: dkey + '/1M/s',
            PresetId: '1351620000001-200030', // hls 1M
            SegmentDuration: '10'
          }
          ,{
            Key: dkey + '/2M/s',
            PresetId: '1351620000001-200010', // hls 2M
            SegmentDuration: '10'
          }
          ,{
            Key: key,
            PresetId: '1351620000001-000010', //Generic 720p - mp4
            ThumbnailPattern: dkey + '-{count}'
          }
        ],
        Playlists: [
          {
            Name: dkey,
            Format: 'HLSv3',
            OutputKeys: [
                dkey + '/600k/s', 
                dkey + '/1M/s', 
                dkey + '/2M/s'
            ]
          }
        ]
    };

    transcoder.createJob(params, function(err, data){
        if (err) {
            console.log(err, err.stack);
            context.fail();
            return;
        }
        context.succeed('Job well done');
    });
};