NgModel not found while create directive - angularjs

Question

Here is my directive fileUpload for validation of image extenstion. I am not getting what is missing here. (Missing Required Controller Controller 'ngModel', required by directive 'fileUpload', can't be found! ) Please if any one can look in to this.
I found so many reference to the same question but no luck . please dont mark this as repeat question.
App.directive('fileUpload', function () {
return {
scope: true, //create a new scope
require: 'ngModel',
link: function (scope, el, attrs,ctrl) {
ctrl.$validators.images = function(modelValue, viewValue) {
console.log(modelValue);
console.log(viewValue);
};
ngModel.$setValidity('extension', true);
el.bind('change', function (event) {
var files = event.target.files;
var validFormats = ['jpg', 'jpeg', 'png', 'gif'];
var validSize = 500000;
for (var i = 0;i<files.length;i++) {
var ext = files[i].name.split('.');
var size = files[i].size;
if (ext !== undefined) {
var isExist = validFormats.indexOf(ext[1].toLowerCase()) >= 0 ? 1 : 0;
if (isExist == true && validSize >= (size)) {
ngModel.$setValidity('extension', true);
} else {
ngModel.$setValidity('extension', false);
}
}
//emit event upward
scope.$emit("fileSelected", { file: files[i] });
}
});
}
};
});
Input :
<div class="form-group has-feedback" style="margin-left: 10px;">
<input name="file" type="file" ng-model="images" accept="image/*" multiple="true" file-upload />
<div style="color:grey; font-size: 12px;">
Extensions : jpg , jpeg , png , gif<br>
Size : 500KB
</div>
<div ng-messages="frmSupportPanelist.file.$error" ng-if="frmSupportPanelist.file.$touched">
<label id="message-required-error" class="validation-error-label" for="file" ng-message="extension">Please select valid Image </label>
</div>
</div>