CSS: Fix upload field display for preview open

Upload Field's dimensions don't work very well on pages when viewed in split view (causes horizontal scroll):

![upload_before](https://cloud.githubusercontent.com/assets/984753/3697795/55dc7062-13b1-11e4-9218-4498eebc3a81.png)

I have adjusted the css so that the field stacks with it's label instead. I tried adjusting the width and padding, but with the number of buttons that need to be displayed at times, it didn't really work. After:

![upload_after](https://cloud.githubusercontent.com/assets/984753/3697812/9b55337c-13b1-11e4-8ccf-5faf731ee4d0.png)