Error when symbolic link is part of listed files

Hi,

I get the following error when rendering a list of files which contains a symbolic link : `array_key_exists(): Argument #2 ($array) must be of type array, null given` (given when redering the list template, on `File::isImage()` call.

This is due to `FileTypeService::preview` not treating the `link` value returned by `SplFileInfo::getType`.

I will provide a PR for that issue.