Choose icon in tree view based on file extension

```
It would be cool to have some more icon variation in the tree view. We don't 
want to inspect file contents, which could be very slow, but we could use a 
simple extension map like we do for file detection in prettify.js.
```

Original issue reported on code.google.com by `dborowitz@google.com` on 12 Nov 2012 at 6:24
