I found that while using the magic files from GnuWin 4.23 worked great for the PECL extension with PHP 5.2 on Win32, when I started migrating to PHP 5.3 all of a sudden Fileinfo no longer worked. Updating the magic files to the latest version (4.26 at the time of this writing) worked like a charm.
I guess the bundled version of Fileinfo uses the new mime magic format while the old PECL version uses the old format.