support for handling file uploads

This adds handling of file uploads by Base64-encoding the content and storing it in the session.
What I mean is that in your test the getPhotoDataBase64Encoded() use the file itself to convert the file in base 64:

```php
public function getPhotoDataBase64Encoded() {
    return base64_encode(file_get_contents($this->photo->getPathname()));
}
```
But we don't have the file yet, right?