How to put flash message into my code?

I have a function

function fileFilter(req, file, cb) {
        if (file.mimetype !== 'image/png' && file.mimetype !== 'image/jpg' && file.mimetype !== 'image/gif' ) {
            console.log('Wrong format!');
            return cb(null, false);
        }
        if ( file.size > 10000 ){
            console.log('Image is too big!');
            return cb(null, false);
        }
        cb(null, true);
}
var upload = multer({ storage: storage, fileFilter: fileFilter });

This function works when I am making post request.

router.post('/changeAvatar', upload.single('avatar'), function(req, res) {
    var id = req.user._id;
    res.redirect('/user/'+id);
});

How can I put flash connect-flash instead my console.log's? Connect-flash is already installed, with cockies and sessions. I tried different ways, but I can't understand phylosophy of flash messages...

via Bim Bam