def show_chosen_image(workdir, index):
    if index >= 0:
        filename = lw_files.currentItem().text()
        workimage.load_image(workdir, filename)
        image_path = os.path.join(workimage.dir, workimage.filename)
        workimage.show_image(image_path)

def choose_workdir():
    try:
        workdir = QFileDialog.getExistingDirectory()
        if workdir:
            return workdir
        else:
            print("Папка не выбрана")
            return None
    except:
        return None

def filter(files):
    result = []
    extension = [".png", ".jpg", ".jpeg", ".gif", ".bmp"]
    for file in files:
        for ext in extension:
            if file.endswith(ext): # endswith("original.jpg") -> .jpg -> []
                result.append(file)
    return result

def show_filenames():
    dir = choose_workdir()
    files = filter(os.listdir(dir)) # ["name.docs", "1.xlsx", "2.pptx", "original.jpg"]
    lw_files.clear()
    for file in files:
        lw_files.addItem(file)

    lw_files.currentRowChanged.connect(partial(show_chosen_image, dir))

btn_bw.clicked.connect(workimage.do_bw)
btn_dir.clicked.connect(show_filenames)