Set `position:fixed` like the other modals

Unlike the other modals the #upload-info box is positioned absolutely, which allows the user to scroll down the `html` element.
![image](https://f.cloud.github.com/assets/2925395/2191794/bc22a820-9853-11e3-83f4-f5af5dd6fd05.png)
html overflow:hidden was disabled so you can see the scroll bar, and the half scrolled page.