node.js app.get not being called

Problem is your static file middleware app.use(express.static(__dirname + '/public')) is "in front" of your router. When you write

app.configure(function () {
app.use(express.bodyParser());
app.use(express.cookieParser());
app.use(express.session({
    store: sessionStore,
    secret: 'secret',
    key: 'express.sid'
}));
app.use(express.static(__dirname + '/public'));
});

this is equivalent to

app.configure(function () {
app.use(express.bodyParser());
app.use(express.cookieParser());
app.use(express.session({
    store: sessionStore,
    secret: 'secret',
    key: 'express.sid'
}));
app.use(express.static(__dirname + '/public'));
app.use(app.router); //Express implicitly appends router middleware!
});

because Express implicitly appends router middleware at the end of stack if you don't add it somewhere explicitly. Now you clearly see that static file middleware is in front of router. If static file is found it is served by app.use(express.static(__dirname + '/public')) and router app.use(app.router) is never called. If you want your request to always pass through router you should put it in front, for example

app.configure(function () {
app.use(express.bodyParser());
app.use(express.cookieParser());
app.use(express.session({
    store: sessionStore,
    secret: 'secret',
    key: 'express.sid'
}));
app.use(app.router); //Now router comes first and will be executed before static middleware
app.use(express.static(__dirname + '/public'));
});

It's because express filters the request before it gets to your code. It finds the file and returns it to the browser.

Solution is either to send an event via socket.io telling the code in user's browser to redirect or move file into private space (outside public directory) and serve it via "fs" as CydGy suggested.