Use binary/octet-stream as the default content-type
Use binary/octet-stream as the default content-type

We encountered a problem with `*.idml` files. In the browsers the
`file.type` is the empty string `''`. Hence, Amazon is sent

```bash
------WebKitFormBoundary3aVHtVCO6rLQpIUz
Content-Disposition: form-data; name="content-type"

------WebKitFormBoundary3aVHtVCO6rLQpIUz
Content-Disposition: form-data; name="file"; filename="example.idml"
Content-Type: application/octet-stream
```

When later downloading the example.idml from S3 one is presented with a
"example.idml.zip" file. Using `binary/octet-stream` as the default
content-type fixes this.

`binary/octet-stream` is also the default content-type used by S3, if
no content-type is passed in the request.
See http://docs.aws.amazon.com/AmazonS3/latest/API/RESTObjectPUT.html

gertfj/s3_direct_upload,gertfj/s3_direct_upload,gertfj/s3_direct_upload