I have written a guideline for this and is available at:

http://cyber7.l1fe.co.za/2011/10/20/pfsense-adding-the-ability-to-ftp-from-lan-to-wan/

Basically the steps are:
In SQUID: add thefollowing in your Custom Options: (NOTE: the ;;; is for new-lines)

;;;acl ftp proto FTP;http_access allow ftp;acl CONNECT method CONNECT;acl ftp_port port 21;;http_access allow ftp_port CONNECT;;;

You also have to add in the Access Control/acl sslports the following: (NOTE: this enables the ports for FTP)

21 50000-50050

The last thing you need to do is disable your System Tunables “debug.pfftpproxy” by changing the “(default) 0″ value to “1″
That is it…

I use FileZilla’s free FTP client and it works like a charm.  Just remember to add your PROXY into the CLIENT.

Kind regards
Aubrey Kloppers
Bible Society of SA