s3.getSignedUrl('getObject', {Bucket: AWS_BUCKET_NAME, Key: 'myObjectsKey', ContentType: 'image/png'}, function (err, url) {
    console.log(err, url);
});

{ [UnexpectedParameter: Unexpected key 'ContentType' found in params]
  message: 'Unexpected key 'ContentType' found in params',
  code: 'UnexpectedParameter',
  time: Thu Dec 18 2014 01:38:19 GMT-0400 (AST) }

StringToSign = HTTP-VERB + "n" +
    Content-MD5 + "n" +
    Content-Type + "n" +
    Expires + "n" +
    CanonicalizedAmzHeaders +
    CanonicalizedResource;