This is because you do not open the Fileinfo extension, simply open it. Open php.ini, find php_fileinfo.dll, the previous # removed, restart apache it.
It is really strange to find that installing filenfo extension on windows is not yet fully explained. Here are the latest most up-to-date steps to make this work:
1. Add php_fileinfo.dll to the list of enabled extensions
2. Download GNU file package for windows
get it from here: http://gnuwin32.sourceforge.net/downlinks/file-bin-zip.php
Extract the folder from the archive called shared\file which contains 4 magic files (magic, magic.mgc, magic.mime, magic.mime.mgc).
3. Place these 4 files in a known place
4. Add system enviroment variable MAGIC that points to the file called magic. For example if you placed magic files in folder called magic in c:\php that means that the propper path is c:\php\magic\magic
5. If you use IIS restart it and fileinfo is ready for use
5a. If you use apache you will have to restart operating system for the service to acknowledge MAGIC env. variable.