Gateway shows file icon for directory called `protocol.ai` instead of directory icon

#### Version information:

```
go-ipfs version: 0.4.12-
Repo version: 6
System version: amd64/linux
Golang version: go1.9.2
```

#### Type:
Bug

#### Severity:
Low
#### Description:

When adding a directory that has a link `protocol.ai`, the gateway shows this directory as a Adobe Illustrator file (I think) rather than a directory.

Steps to reproduce:

```
$ mkdir test
$ mkdir test/protocol.ai
$ mkdir test/libp2p.io
$ echo "protocol.ai" > test/protocol.ai/index.html
$ echo "libp2p.io" > test/libp2p.io/index.html
$ ipfs add -rQ test
QmQ9C4WiXrPE2NUW8GM3GwXdZudvxzJynMM7XwzBxi3CeX
```

Looking at the gateway on the test directory (https://ipfs.io/ipfs/QmQ9C4WiXrPE2NUW8GM3GwXdZudvxzJynMM7XwzBxi3CeX) you'll see the following:


Notice the `protocol.ai` link shows some file extension icon instead of the directory icon.

Edit: it still works as a directory though, as you can navigate to https://ipfs.io/ipfs/QmQ9C4WiXrPE2NUW8GM3GwXdZudvxzJynMM7XwzBxi3CeX/protocol.ai/ and see the right output. But the icon for the directory is wrong.