Add extension

What else I need to do to add extension.
I added it to config.extensions.php in system folder but it is not working.

I forgot to mention this extension has more than 3 symbols.
Extension that is *.wotreplay.

Where I have to change it to get it working?

thanks in advance

Hi,
I made it, just exchanged line 143 in pfs.inc
from
 

$f_extension = mb_substr($u_name, $dotpos, 5);

to

$f_extension = mb_substr($u_name, $dotpos, 9);

for change number of characters after the dot and all working well.
Thanks anyway.