In case anyone is wondering how to fix this under FreeBSD:
in /usr/ports/lang/php5, create a Makefile.local file with this line
CONFIGURE_ARGS+=--enable-fileinfo
and recompile php5.
Source: TIV.NET: Enabling internal PHP fileinfo on FreeBSD (http://www.tiv.net/2010/11/enabling-internal-php-fileinfo-on.html)