Calculating a hash of the file's content while it uploads

I want to generate a hash of the file's content while it is being uploaded. Can I hook in and observe the [file stream](https://github.com/expressjs/multer/blob/master/storage/disk.js#L40) somehow? And then possibly stow this information somewhere? [This other issue is asking for something similar](https://github.com/expressjs/multer/issues/211).

Here's what I'd like to do with the stream:

``` javascript
var crypto = require('crypto');
var path = require('path');
// ...
var hash = crypto.createHash('sha256');
file.stream.on('data', function(chunk) { hash.update(chunk) })
file.stream.on('end', function() {
var sha256 = hash.digest('hex');
// ... look it up in the database, etc
})
```