Roku

This plugin makes uploaded files only accessible by logged-in users.

Uploads Protector

This plugin makes uploaded files only accessible by logged-in users.

Onni Hakala

Geniem Oy

WordPress plugin which removes all accented characters like <code>åöä</code> from future uploads and has easy wp-cli command for removing accents from current uploads and attachment links from database. 
This helps tremendously with current and future migrations of your site and helps you to avoid strange filename encoding bugs.
Sanitize accents from Cyrillic, German, French, Polish, Spanish, Hungarian, Czech, Greek, Swedish. 
This even removes rare but possible unicode NFD characters from files by using <a href="http://php.net/manual/en/normalizer.normalize.php" rel="nofollow ugc">PHP Normalizer class</a>. These usually happen if you have mounted uploads into your vagrant box in OS-X.
This plugin is wordpress multisite compatible.
More information in github page: https://github.com/devgeniem/wp-sanitize-accented-uploads

WP Sanitize Accented Uploads

Simple plugin which removes accented characters from uploaded files.

Uploads Protector vs WP Sanitize Accented Uploads