The mime_type function doesn’t handle unknown extensions

The `mime_type` function [doesn’t check](https://github.com/sofadesign/limonade/blob/master/lib/limonade.php#L2415) if an extension exists before getting its type. So if someone call this function with a non-existing extension (e.g. an empty string), he/she gets a warning: 

```
Notice: Undefined index:  in /…/sofadesign/limonade/lib/limonade.php on line 2417
```
