So, The thing is I want to share some files with my friend. Now Both of us use different networks and we are behind NAT firewalls. I downloaded other people's files downloading seems to be working. But will it work on our case?

it should. But the only way to know for sure is to try.

to find your friend, or for him to find you, you could try searching for something you know the other is sharing.

or one of you make a private channel and then send the link to the other via email or something.

or both join an already running channel and add each other to the contacts list.

It failed. Sharing files should have worked. My shared files are not shown on search.

did you try both joining the same channel?

as soon as you find your friend be sure to add him to your contacts list.