Trouble with S3.putObject
I'm not sure if this is an issue with the apollo-upload-server, s3.putObject, or my code; but I'm hoping someone can get me pointed in the right direction.

I cloned the repo from https://github.com/jaydenseric/apollo-upload-examples and am running the client. I made a few tweaks to the client but largely the same. I'm trying to do some validation on a csv that gets uploaded, if it passes validation I would like to upload the file to s3. I posted an issue with some code on [stackoverflow](https://stackoverflow.com/questions/49223337/how-to-s3-putobject-stream-from-apollo-upload-server).

I find it surprising that you need to specify a file length for a stream, isn't the whole point that we don't know the length until it's done? The files that I'll be uploading are too big to keep in memory (multiple GB) so a stream seems like my only option here. I'm wondering if there is some substantial difference between the stream produced by the upload-server and `fs.createReadStream()`   
