custom ETag function isn't working for SendFile

I set the custom function according to the http://expressjs.com/en/api.html#etag.options.table
```
app.set('etag', function (body, encoding) {
return generateHash(body, encoding) // consider the function is defined
})
```
Despite on that it's not working and the best way I could influence etag value is to call
sendFile with etag : true or false but this just turns on and off the standard etag (also not documented) , not the custom one.

https://github.com/expressjs/express/issues/2294
https://github.com/expressjs/express/issues/2129

My use case is that I want to track responses of specific files. So I wanted to make a "strong hash" that I can be 100% sure would unambiguously represent the file.