Removing the refernce to the file from the fields using batch API

Issue #1426 

![edit node screen](https://f.cloud.github.com/assets/1222368/495102/d952509c-bbb5-11e2-8a68-76192e470d02.png)
![front page with file](https://f.cloud.github.com/assets/1222368/495103/dc91f474-bbb5-11e2-9ca5-90457cdfec4f.png)
![empty-field](https://f.cloud.github.com/assets/1222368/495104/df5938c0-bbb5-11e2-8538-a8d2bb536c2b.png)
![no-trace](https://f.cloud.github.com/assets/1222368/495105/e203e2fa-bbb5-11e2-8f08-f21c614c7655.png)