Can not post image / file / video on wall. It says "Forbidden - You don't have permission to access /activity/post/ on this server" ?
Solution :
You need to disable PHP Mod Security on your server via .htaccess
Open .htaccess file and insert this code
<IfModule mod_security.c>
SecFilterEngine Off
SecFilterScanPOST Off
</IfModule>
Save htaccess. But that code doesnt work to some servers. So you need contact your hosting provider support to disable it.