Restangular won't change Http Content-Type
As said in comment above and According to docs you should pass headers as fourth parameter. So your code should be
$rootScope.objects.user.withHttpConfig({ transformRequest: angular.identity }).customPUT( $scope.objects.user, undefined, undefined, { 'Content-Type': "multipart/form-data" } ).then(function(resp) { if (resp == "OK") { $scope.successes = [{ msg: "Saved" }]; } }, function(err) { console.log(err); $scope.errors = err.data.errors; });Reference url : https://github.com/mgonto/restangular#custom-methods