Johannes Schluter has pointed out a "hidden gem" for PHP 5.3 in his latest blog post - the fileinfo extension.

This series is not meant to be complete but some personal choice, these blog postings are also no replacement for documentation, but just pointers. My goal is that you try out 5.3 right now so we can fix bugs as soon as possible before releasing it The NEWS file has a quite short entry for my first subject: Added fileinfo extension as replacement for mime_magic extension. (Derick)

He looks at one situation where the extension could come in very handy - getting the content type of the file for a download push - as well as why its better than the older mime_content_type function.