Streamed file size is 5-6 times bigger than the original size
I am using apollo-upload-server with graphql-nodejs boilerplate [template](https://github.com/graphql-boilerplates/node-graphql-server/tree/master/advanced), and apollo-upload-client implementation with react native (expo). 

While uploading the files from filesystem, the filesize on the server is at least 5-6 times the size in the local system. I do not know what is the exact cause of the issue. Here are the steps to reproduce:

In Expo, I am returning the local uri from ImagePicker and creating a file to upload:
`const picture = new ReactNativeFile({
      uri: returnedURI,
      type: 'image/jpg',
      name: 'somename.jpg',
    });`

and then running a mutation as follows:
```
<Mutation mutation={uploadFileMutation}>
                {uploadFile => (
                  <Button
                    styleName="clear"
                    style={{ flex: 1 }}
                    onPress={() => {
                      uploadFile({
                        variables: {
                          picture,
                        },
                      });
                    }}
                  >
                  </Button>
                )}
              </Mutation>
```

On the server-side, upload is processed as follows: 
```
const processUpload = async upload => {
  const { stream, filename } = await upload
  const { id, path } = await storeUpload({ stream, filename })
  return path
}

const storeUpload = async ({ stream, filename }) => {
  const id = nanoid()
  const path = somepath

  return new Promise((resolve, reject) =>
    stream
      .pipe(createWriteStream(path))
      .on('finish', () => resolve({ id, path }))
      .on('error', reject),
  )
}

const file = {
    async uploadFile(parent, { picture }, ctx, info) {
      const resolvedURL = await processUpload(picture)
      const url = `http://localhost:4000/${resolvedURL}`
      return ctx.db.mutation.createFile(
        {
          data: {
            url,
          },
        },
        info
      )
    },
}
```

Here are two images for comparison - one from the filesystem, and second uploaded one

**From filesystem- Size: 45kb**
![81fceb8e-2352-40cd-a10a-c77084fd8a08](https://user-images.githubusercontent.com/33306256/41337550-215c1f84-6f0d-11e8-848f-2fa0d2dd3fc3.jpg)
**Uploaded copy - Size: 257kb**
![iyvmzwjig0coq4bnunbqh](https://user-images.githubusercontent.com/33306256/41337606-53292d54-6f0d-11e8-8d9c-26a9c28fc8c9.jpg)



