Has Mountain Lion changed file sharing setup or permissions? Recently I installed Mountain Lion (I had Snow Leopard), I used to share files with a PC enabling the file sharing and use Filezilla in the PC to download and upload files.

Now with Mountain Lion for some reason I can't connect to the Mac computer. Nothing changed; I'm using the same credentials. This is exactly what I did: http://www.dummies.com/how-to/content/how-to-set-up-file-sharing-from-mountain-lion.html

Is there some kind of new security policy or something similar in Mountain Lion?
The FTP server in Mountain Lion is still there, just not part of the GUI. You can re-enable it using the following in Terminal:

sudo -s launchctl load -w /System/Library/LaunchDaemons/ftp.plist


To disable, do unload instead of load.

*The FTP root directory is your Users home folder.