I am about to write how installed this package.
First of all, I tried with "pear install fileinfo" - as the manual says.
But the pear command said that 'Package "Fileinfo" is not valid,
Then the "pear install pecl/fileinfo" was a better way. But at that time the "phpize" command was missing.
I installed that (on openSUSE distributions it is in the php5-devel, but I think you can find it in your distro's corresponding php-devel package).
After that you may install "re2c" (I did). It's homepage is: http://sourceforge.net/projects/re2c
Copy the magic file of Apache (usually in /etc/apache2) into the following directory: /usr/locale/share/file/ or /usr/share/file/
Then you have to install "libmagic-dev". If you have Debian based system you can simply install it with apt.
But if you have an rpm based system (like me), you have to download the following package: http://packages.debian.org/unstable/libdevel/libmagic-dev
It contains the files we need.
So, download the file, browse it with Midnight Commander (mc) (you have to apt and dpkg be installed) and simply extract (so copy) the /usr folder (it is inside the CONTENTS folder) of the .deb package to the root folder.
And now give the "pear install pecl/fileinfo" command another try :)
Ps: Don't forget to check whether the script has wrote the following line into the php.ini (on openSUSE: /etc/php5/apache2): extension=fileinfo.so
I hope, I could help.