Feature/removefile - allows the files field type to be cleared

I was having clients ask for this quite a bit. They wanted to remove images after uploading/assigning them.

Clearing it basically just reverts the field back to its dummy state.

If the file is required it will not submit after being cleared. You will have to upload a new file.

![Screen Shot 2013-02-28 at 2 57 31 PM](https://f.cloud.github.com/assets/1425304/207026/457cb1b8-81e1-11e2-8680-e425955edbd4.png)