Added file type field for object File on server side

Object File on client side has "type" field which indicate file extension (screenshot from dev tools below).
![siofu-client](https://cloud.githubusercontent.com/assets/4158258/4337310/3878d374-400e-11e4-97c6-0d92cef61919.png)
But on server side object File doesn't have "type" field:
![siofu-server](https://cloud.githubusercontent.com/assets/4158258/4337266/b3d99482-400d-11e4-986a-e8bb940fd938.png)

Via using this field we will be able to handle the file depending on its' type.