mime_content_type returning text/plain for css and js files only

I'm encountering this weird issue with PHP's mime_content_type, it works fine, except for CSS and JavaScript files, which it returns text/plain for (PHP's default mime is set to do text/html).

Modifying the mime.types file seems to make no difference (its path on httpd.conf is correct).

Examples

<?php
  echo(mime_content_type('index.html')); // returns text/html
  echo(mime_content_type('default.png')); // returns image/png
  echo(mime_content_type('bootstrap.min.js')); // returns text/plain
  echo(mime_content_type('css/animate.css')); // returns text/plain
  echo(mime_content_type('css/style.css')); // returns text/x-asm
?>

The same exact files served on the same exact Apache installation gets correctly marked on their Content-Type headers, it's almost as if PHP's using its mime detection mechanism.

I'm using Wamp 3.0.6 64-bit and the built-in PHP 7.0.10 on Windows 10 64-bit build 15063.483 mod_mime and mode_mime_magic are enabled.