如何在Tkinter中只打开一种格式的文件

所以我想做一些类似记事本的东西,但我想打开 .txt 仅限文件。
我不知道怎么做,也没有在网上找到任何东西。
我的代码:

import tkinter as tk # Import Tkinter
from tkinter.filedialog import askopenfilename # Import dialog box (to ask for file directory)

window = tk.Tk() # Create window
window.wm_geometry("600x600") # Set geometry
window.title("Test") # Rename

class FileOperations: # Class for file operations

    def open_file(self, path): # File open function
        file_opened = open(path, "r") # Open
        file_contentment = file_opened.read() # Read
        return file_contentment # Return

file_ops = FileOperations() # Assign variable to a class

newfile_button = tk.Button(master=window, text="New", width=10, height=1, font=("Arial", 10)) # Not done yet, do not mention it
newfile_button.grid(column=0, row=0) # Grid

savefile_button = tk.Button(master=window, text="Save File", width=10, height=1, font=("Arial", 10)) # Not done yet, do not mention it
savefile_button.grid(column=1, row=0) # Grid

openfile_button = tk.Button(master=window, text="Open File", width=10, height=1, font=("Arial", 10),
                            command=lambda: file_ops.open_file(askopenfilename())) # Fix it?
openfile_button.grid(column=2, row=0) # Grid

tk.mainloop() # Mainloop

任何建议都很有帮助。