How to change value directive from factory?

I have a factory and I want to change value of input. How could I do this ?

factory

var app = angular.module('starter');

app.factory('CameraFactory', ['$q', function($q, ngFileModel) {

var options = {
    quality: 50,
    destinationType: Camera.DestinationType.FILE_URI,
    sourceType: 0,      // 0:Photo Library, 1=Camera, 2=Saved Photo Album        
}

var onSuccess = function(imageData) { 
	window.resolveLocalFileSystemURI(imageData, function(fileEntry) {                
        //console.log(fileEntry.nativeURL);                           
        ngFileModel.setPath(fileEntry.nativeURL);
    });    

    var image = document.getElementById('smallimage');
	image.src = imageData;  

};

var onFail = function(e) {
    console.log("onFail! " + e);	    
};
return {
    getPicture: function() {
     	return navigator.camera.getPicture(onSuccess,onFail,options);
}
 }
}]);

directive

var app = angular.module('starter');

app.directive('ngFileModel', function(){

	return{
		restrict:'A',
		link: function(scope, element, attrs){
			var field = attrs.ngFileModel;

			scope.$watch('setPath', function(filePath){
				field.value = filePath;
			});
		}
	};
});

html

<div>          
  <img ng-src="img/ionic.png" id="smallimage" width="150" heigth="150">
  <input type="file" ng-model='User.imageFile' id='imageFile' ng-file-model/>
  <button class="button button-stable"  ng-click="selectImage();">Imagem</button>