Hi,
Thanks for this usefull project.
I'm not able to code, so I have ask to Codex and he has edited helper.php (see next post link)

I'm not able to say if it's secure and well coded, but it works.
It will be nice to include it in Perlite.
Viewable files extensions are in environment variable :

• TREE_VISIBLE_EXTENSIONS=pdf,docx

Regards,
Meta