Occluding Mimes

Looks like the missing file extension for the  `application/pgp-encrypted` mime type is causing `Mime.ext_from_type("") == "application/pgp-encrypted"`.

Also, the `.mp4` mapping isn't unique:

```
video/mp4       .mp4
application/mp4 .mp4
```


I'm happy to add a proper fix to this PR if there's a particular approach you'd like to take for non-unique mappings.


(also worth noting that since the mimes are stored in a txt file, changes to it don't cause elixir to recompile `mime.ex` between runs)