How to generate thumbnails of a bitmap using AsyncTask?

I have a FragmentActivity that has a listView, in each item there is a button to open a Dialog, in this Dialog another listView is executed with an adapter that has a TextView and an ImageView, the list comes from a directory of photos, wanted put these photos as thumbnails, those thumbnails, I managed to do this without using AsyncTask, however, it got a bit heavy because I have to compress the bitmaps, according to my studies, the best way is to use an AsyncTask to perform the heavy process , I did this and it runs, however, the bitmap does not appear in the respective ImageView. I'll put the code in order.

DialogFragmentListPhotos.class

public class DialogFragmentListFotos extends DialogFragment {

private ListView listView;
private String path, title;


public static DialogFragmentListFotos newInstance(Bundle bundle) {
    DialogFragmentListFotos f = new DialogFragmentListFotos();
    f.setArguments(bundle);
    return f;
}

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setStyle(STYLE_NORMAL, android.R.style.Theme_DeviceDefault_Light_Dialog);
    path = getArguments().getString("path");
    title = getArguments().getString("title");
}

@Override
public Dialog onCreateDialog(Bundle savedInstanceState) {
    Dialog dialog = super.onCreateDialog(savedInstanceState);
    String fotosDaOcupacao = getActivity().getString(R.string.fotosDaOcupacao);
    dialog.setTitle(fotosDaOcupacao + " " + title);
    return dialog;
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    final View view = inflater.inflate(R.layout.dialog_list_fotos, null);

    ListView listView = (ListView) view.findViewById(R.id.listView_fotos);
    Button cancelar = (Button) view.findViewById(R.id.button_cancel_list_fotos);

    final File file = new File(path);
    final File[] files = file.listFiles();
    ArrayList<String> stringArrayList = new ArrayList<String>();
    ArrayList<File> fileArrayList = new ArrayList<File>();
    for (int i = 0; i < files.length; i++) {
        stringArrayList.add(files[i].getName());
        fileArrayList.add(files[i]);
    }
    //ArrayAdapter<String> filesArrayAdapter = new ArrayAdapter<String>(getActivity(), android.R.layout.simple_selectable_list_item, theNamesOfFiles);
    ArrayAdapter<File> filesArrayAdapter = new ArrayAdapterFotos(getActivity(), R.layout.adapter_d_list_fotos, fileArrayList);

    cancelar.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            getDialog().dismiss();
        }
    });

    listView.setAdapter(filesArrayAdapter);
    listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
        @Override
        public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
            File file = new File(path + "/" + files[position].getName());
            String type = getType(file);
            Intent it = new Intent();
            it.setAction(android.content.Intent.ACTION_VIEW);
            it.setDataAndType(Uri.fromFile(file), type);

            try {
                getActivity().startActivity(it);
            } catch (Exception e) {
                String appNessNaoEncontrado = getActivity().getString(R.string.appNessNaoEncontrado);
                ToastManager.show(getActivity(), appNessNaoEncontrado, ToastManager.ERROR, ToastManager.LENGTH_LONG);
            }
        }
    });

    return view;
}

private String getType(File file) {
    String type = null;
    try {
        URL u = file.toURL();
        URLConnection uc = null;
        uc = u.openConnection();
        type = uc.getContentType();
    } catch (Exception e) {
        e.printStackTrace();
    }
    return type;
}
}

ArrayAdapterPhotos.class

public class ArrayAdapterFotos extends ArrayAdapter<File> {

private final int resourceId;
private ArrayList<File> files;
private Context context;
private int pos;
public ImageView imageView;

public ArrayAdapterFotos(Context context, int resource, ArrayList<File> files) {
    super(context, resource, files);
    this.resourceId = resource;
    this.files = files;
    this.context = context;
}

@Override
public View getView(int position, View convertView, ViewGroup parent) {
    Log.i("Posição", String.valueOf(position));
    if (convertView == null) {
        convertView = LayoutInflater.from(getContext()).inflate(resourceId, parent, false);
    }
    pos = position;
    imageView = (ImageView) convertView.findViewById(R.id.imageView_foto_thumbail);
    TextView textView = (TextView) convertView.findViewById(R.id.textView_nome_foto);
    textView.setText(files.get(position).getName());

    BitmapTask bitmapTask = new BitmapTask();
    bitmapTask.execute(files.get(position).getAbsolutePath());

    return convertView;
}

public class BitmapTask extends AsyncTask<String, Bitmap, Bitmap> {

    private String data;

    @Override
    protected Bitmap doInBackground(String... params) {
        int h = 48; // height in pixels
        int w = 48; // width in pixels
        data = params[0];
        Bitmap bitmap =       ThumbnailUtils.extractThumbnail(BitmapFactory.decodeFile(data), h, w);
        publishProgress(bitmap);
        return bitmap;
    }

    @Override
    protected void onProgressUpdate(Bitmap... values) {
        imageView.setImageBitmap(values[0]);
    }

    @Override
    protected void onPostExecute(Bitmap result) {
        imageView.setImageBitmap(result);
    }
 }
}