Sort fileadmin items by name additionally

Hi mrjoes,

I would like to show the files order by name additionally using `File Admin`. Seems it only topped directories according to the `flask_admin/contrib/fileadmin.py` as below:

```python
items.sort(key=itemgetter(2), reverse=True) 
```
But I think `itemgetter(2, 1)` would be better since it also sorts by name, and makes better readability for the file list.

How about you? Hope this patch makes sense and helps.

Cheers