fix #978 uploading and extracting a zip file from FileExplorer.js

When dragging a zip file, FileExplorer will give you a target to allow uploading and unzipping the file.
the target won't show up if the dragged file isn't a zip file only in Chrome. Both IE and FF don't tell you what the dragged files are until they are dropped so will always display the target.

![fs](https://f.cloud.github.com/assets/645740/2478907/7b7972c8-b08a-11e3-850d-978470174d84.jpg)