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Some poeples don't want to show thumbnails, while others want. 

So, why not give the option?

The preview link finally work, but why thumbnail preview doesn't appears when I disable files cannot be downlodable in permission?

In  the doc it's say ...

Simply add the following line inside the configuration file (customizables/config.php):

But this config file doesn't exist

Is it possible to know where CSS for thumbnails are?

When I desactive the folder CSS, there's no change on thumbnails