parse file in angularjs controller before upload

I have angularjs application where user should be able to import data from text files. Before upload, I would like to read several lines from file on a client side to do some validations. I am using https://github.com/nervgh/angular-file-upload to upload file. I can upload file and do validation on a server side but files can be few megabytes big and I do not want to create unnecessary traffic if data are invalid.

Inside my controller I have following code:

$scope.validateFile = function(file){

  console.log(file.name);
  console.log(file.size);      
  var reader = new FileReader();

  reader.onloadend = function(evt){      
    console.log(evt.target.result);
    //do something with file content here 
  };

  //var blob = file.slice(0, file.size - 1);
  reader.readAsText(file);

};

When executed, console outputs file name and size. After that I got follwoing error:

TypeError: Failed to execute 'readAsText' on 'FileReader': parameter 1 is not of type 'Blob'.

When I remove comment from the following line...

var blob = file.slice(0, file.size - 1);

and change the last line to be:

reader.readAsText(blob);

this is the error from console

TypeError: undefined is not a function

It looks like that slice function is not working. Why slice() doesn't work? Is it possible to do it like this?

UPDATE:

Based on DTing's comment I found an error

My original html was

<input type="file" nv-file-select="" uploader="uploader" multiple  /><br/>
...   
<tr ng-repeat="item in uploader.queue">
...   
ng-click="validateFile(item.file)"

I have changed last line to

ng-click="validateFile(item._file)"

Now it works.