fileService.js

//file service
angular.service('FileDialog', function($q){

  //reads file from disk, returns a promise (integrated into the digest cycle)
  function readFile(filename){
    return $q(function(resolve, reject){
      fs.readFile(fileName, 'utf-8', function (err, data){
        if(err){
          reject(err);
          return;
        }
        resolve(data);
      });
    });
  }
  
  function openFileDialog(){
    return $q(function(resolve, reject){
      dialog.showOpenDialog(function(filename){
        resolve(filename);
      });
    })
  }

  this.openFile = function(){
    return openFileDialog().then(readFile);
  }
  
});

usage.js

angular.controller('Foo', function(FileService){
  
  this.openFile = function(){
    FileService.openFile().then(function(fileData){
      //now do something with file....
    });
  }
  
});