You need FTP server software. There's several of them available, check Tucows for example.
Yes, there's some risk involved. If you're not careful people will start to use your harddisk to store illegal content (child porn, pirated software, etc. etc.) that they don't want on their own machines.
So you'd better be careful about how you set permissions on your server.
Your ISP may also not allow you to host an ftp server (or any server) on your connection, in which case doing what you want can get your account suspended.
As to how to get the laptop seen outside the LAN, that's a question of DNS record propagation which I'm not familiar with (not being a network admin). Not even sure you can do that yourself without the help of your ISP.
jwenting
duckman
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