fileFilter error, and using csurf

My csrf protection middleware, using "csurf", is configured after multer, as it should (I guess).

If I have an error, for example, in the fileFilter function :
```app.use(multer({ storage: fileStorage, fileFilter: fileFilter }).single('image'));```

Then, it goes straight to the express error-handling middleware :

```
app.use((error, req, res, next) => {
res.status(500).render('500', {
pageTitle: 'Error!',
path: '/500',
isAuthenticated: req.session.isLoggedIn
});
});
```

But in the page I render, I use the csrf token defined in a previous middleware, so it's undefined ('invalid csrf token').

How can I then use multer with csurf, for those errors ?

Here's the fileFilter function :

```
const fileFilter = (req, file, cb) => {
if (
file.mimetype === 'image/png' ||
file.mimetype === 'image/jpg' ||
file.mimetype === 'image/jpeg'
) {
cb(null, true);
} else {
cb('INVALID FILE!!!!', false);
}
};
```

As I said, it is triggered before the request is handled by the csrf middleware, hence the issue.