AngularJS - Why is not included in scope form validation

Question

I'm not really sure if this has already been answered since I really can't find the answer in other posts.

I have an array of file names (ex. [flower.jpg, rose.jpg]) that I want to validate against the chosen files from the input type=file form. This validation detects duplicates then I want to set the error validity like this "$scope.detailsForm.files.$setValidity("duplicate", true);".

The problem is "$scope.detailsForm.files" is getting undefined but this is not the case with other input types such as "input type="text" name="username"".

What I've noticed using AngularJS Batarang I am seeing that the scope for the "detailsForm" contains all my inputs' "name" attribute except for my input type="file"

Also what's weird is adding "ng-model='anythingHere'" to my input type="file" makes the "name" attribute exist in the "detailsForm" scope, thus achieving my desired result.

However it really is bugging me because I really don't want to add the ng-model directive since I'm already using angular-file-upload for this.

Here's a snippet of the codes:

Controller:

if (imageArr.indexOf(uploadFilename) != -1) {
   $scope.detailsForm.files.$setValidity("duplicate", true);
   return;
}
else {
   $scope.detailsForm.files.$setValidity("duplicate", false);
}

HTML:

input  type="file" name="files" ng-file-select="onFileSelect($files)

Hope you understand my question and situation.

PS: This is actually my first time posting here in stackOverFlow since the posts here has always answered my questions. This time I really need help! Masters I need chu!