Hi, you need to install PHP extension called fileinfo. You can ask your hosting provider to do that for you, or if you are managing your own server you can do it yourself.

http://www.php.net//manual/en/fileinfo.installation.php

Check this link, maybe will help a bit. I had same issue, but my company had cpanel server so it was fairly easy to install as PECL package from WHM administration.

easy... after blow my head searching... lol

well you just need to modify the php.ini file and uncomment the next line:

;extension=php_fileinfo.dll

so the final line is

extension=php_fileinfo.dll

DONE! restar apache.

retry the "composer update" command and continue with the installation

http://image.intervention.io/getting_started/installation

Thanks vanmart. It did save me some time.