(DRF 3.0) Added missing default style for FileField

The browsable api forms were missing file inputs for ```FileField```. This was easily added by including the information in ```HTMLFormRenderer.default_style```

Before:
![screen shot 2014-11-12 at 1 10 46 pm](https://cloud.githubusercontent.com/assets/1307950/5018630/5e63b426-6a6d-11e4-9bd3-cb549747a303.png)

After: 
![screen shot 2014-11-12 at 1 11 29 pm](https://cloud.githubusercontent.com/assets/1307950/5018640/713f75e4-6a6d-11e4-81ce-55e2d42f1eb0.png)