if you change the 'path' from '/tmp' to something else, you need to make change
in file 'session.php' in APPPATH/modules/crude/config/session.php as well.
The install instruction does not mention that.
Fileinfo must be included by default in PHP 5.3+.
If you don't have it, the person responsible for your PHP binary has probably used a pre 5.3 compile script, which doesn't contain the correct includes.
This happens a lot with hosters that upgraded to 5.3, but use a custom compiled binary, and couldn't be bothered to update it.
edit: if you're on Windows, the dll is probably missing.
Go to http://pecl4win.php.net/ to get the php_fileinfo.dll if your PHP installation didn't come with it, and you haven't installed the Extensions package. Then make sure you have extension=php_fileinfo.dll somewhere in your php.ini, and restart your web server.