Handling events after promisify using busboy

I'm trying to change the flow from using callbacks to promises, so far this is what I had done: Using bluebird:

var q = require('bluebird');
var busboy = q.promisifyAll(require('connect-busboy'));

Normally using callbacks what I do is the following

req.busboy.on('file',function(fieldname, file, filename, encoding, mimetype){
    console.log('uploading file:'+mimetype);
      file.on('data',function(data){
      console.log('gettting data ');
    });
    file.on('end', function(){
      console.log('file end');
    });
}

What I'd like to do is the following:

req.busboy.on('file').then(function(fieldname, file, filename, encoding, mimetype){
    console.log('uploading file:'+mimetype);
    file.on('data').then(function(data){
      console.log('gettting data ');
    }).on('end').then(function(){
      console.log('file end');
    });
});

I'm getting the following error:

Object # has no method 'then'