This is useful for single page apps where the routing is handled on the
client. I'm using it to serve a webapp using `dodrio`.

Only return the root when the requested file doesn't have an extension.

If the file has an extension, it probably is a file that we should 404.

Adding the extension requirement would break urls like

```
/a/path/with/the/last/part/with.a.dot
```
which might come up in user-created URLs.