Look under the Extended Privileges tab of Manage > Security. It's the presence or absence of the "fmwebdirect" or "fmiwp" extended privilege that determines who can access the file.
If you had opened the file with FM13 (Pro or Advanced), it would have created the missing privilege for you. But you can manually add it.
Note that FM13 no longer automatically creates the fmiwp privilege, but if you open a FM13 file in FM12 it will add it.
Don't forget to assign the extended privilege to a privilege set. That's why the sample file didn't show up in your IWP list in the web browswer.