Add ability to download certain file types

### Describe the problem

Right now, there isn't a way to download files with an anchor like this:

```
<a href="flutter-rules.txt" download>Download Flutter Rules</a>
```

Unless there are any big issues, can we add the ability to do this? It would good to have a list of filetypes that allowed.

@ericwindmill looping you in since you have a potentially larger request.