mime_content_type returning text/plain for css and js files only
The PHP doc is pretty explicit about that:
Returns the MIME content type for a file as determined by using information from the magic.mime file.
PHP and Apache don't use the same mime databases. Apache uses mime.types but PHP uses magic.mime (can't remember where it's located, /etc/ on a Unix system, I think)
No sure how to edit it on Windows but here are some tips for Linux: How to create a custom magic file database