aws-sdk node.js s3 putObject from request body

I'm trying to save a PDF into S3 with the AWS-SDK. I'm getting the PDF through the body of a POST (Application/PDF).

When saving the file into the local HD with fs.writeFile the file look OK. But when uploading into s3 the file is corrupted (single white page pdf)

Any help or hint would be greatly appreciated!

var data = body // body from a POST request.
var fileName = "test.pdf";

fs.writeFile(fileName, data, {encoding : "binary"}, function(err, data) {
    console.log('saved'); // File is OK!
});

s3.putObject({ Bucket: "bucketName", Key: fileName, Body: data }, function(err, data) {
    console.log('uploaded') // File uploads incorrectly.
});

EDIT:

It works if I write and then read the file and then upload it.

fs.writeFile(fileName, data, {encoding : "binary"}, function(err, data) {
    fs.readFile(fileName, function(err, fileData) {
        s3.putObject({ Bucket: "bucketName", Key: fileName, Body: fileData }, function(err, data) {
            console.log('uploaded') // File uploads correctly.
        });
    });
});