Upload file with AngularJS and Express.js

I'm trying to upload files using AngularJS and Node.js (with Express.js). I'm using danialfarid/angular-file-upload. In my view (I use jade) I have

label Upload avatar
input(type="file" ng-file-select='uploadAvatar($files)' accept="image/png, image/jpeg")
span(ng-show='uploadInProgress') Upload progress: {{ uploadProgress }}
img(ng-src='uploadedAvatar' ng-if='uploadedAvatar')

Here's the uploadAvatar function in my controller (I use CoffeeScript in the frontend, here is the compiled javascript version):

$scope.uploadAvatar = (image) ->
  if angular.isArray image
    image = image[0]

  if image.type isnt 'image/png' and image.type isnt 'image/jpeg'
    alert('Only PNG and JPEG are supported')
    return

  $scope.uploadInProgress = true
  $scope.uploadProgress = 0

  $scope.upload = $upload.upload({
    url: '/api/users/avatar',
    method: 'POST',
    file: image
  }).progress( (event) ->
    $scope.uploadProgress = Math.floor(event.loaded / event.total)
    $scope.$apply()
  ).success( (data, status, headers, config) ->
    $scope.uploadInProgress = false
    $scope.uploadedAvatar = JSON.parse(data)
  ).error( (err) ->
    $scope.uploadInProgress = false
    alert "error"
    console.log "error: #{err.message || err}"
  )

Here is my route:

app.post('/api/users/avatar', middleware.auth, users.uploadAvatar);

And the problem is that in my users.uploadAvatar controller I cannot access the file:

exports.uploadAvatar = function(req, res, next) {
  var currentUser = req.user;
  console.log(req.files); // Output is undefined
  // Some other code
}