AttributeError: 'str' object has no attribute 'policy'

I am new to Python. I am trying to make an email script that can send an email. First, I created a Python script without any classes, just executing functions to make sure the script works as expected. After I got the expected output. I am trying to rewrite the script classes to find out. But I am getting an error that I don't understand. I don't understand where the problem is actually.

Below is the code and also a screenshot of the error

import smtplib
import os
import sys
import mimetypes #for guess mime types of attachment

from email import encoders
from email.mime.multipart import MIMEMultipart
from email.mime.text import MIMEText
from email.mime.audio import MIMEAudio
from email.mime.base import MIMEBase
from email.mime.image import MIMEImage

class Email(object):
  message = None
  subject = None
  from_address = None
  to_address = None
  body = None
  email_server = None
  attachment = None

  def __init__(self,from_address,to_address,subject,body,attachment,email_server):
    self.message = MIMEMultipart()
    self.message['subject'] = subject
    self.message['From'] = from_address
    self.message['TO'] = to_address
    self.body = MIMEText(body, 'plain')
    self.message.attach(body)
    self.email_server = email_server

    if attachment is not None:
      self.attachment = attachment
      self.attach_attachment()

  def get_message(self):
    return self.message

  def send_message(self,auth):
    username, password = auth.get_user_auth_details()

    server = smtplib.SMTP(self.email_server)
    server.starttls() #For Encryption
    server.login(username, password)
    server.send_message(self.message)
    server.quit()

  def attach_attachment(self):
    self.messaege = self.attachment.set_attachment_type(self.message)



class Security(object):
  username = ""
  password = ""

  def __init__(self,username, password):
    self.username = username
    self.password = password


  def get_user_auth_details(self):
    return self.username, self.password

class Attachment(object):
  attachment_path = ''

  def __init__(self,attachment_path):
    self.attachment_path = attachment_path

  def is_directory(self):
    return os.path.isdir(self.attachment_path)

  def is_file(self):
    return os.path.isfile(self.attachment_path)

  def guess_and_get_attachment_type(self, filenamepath):
    ctype, encoding = mimetypes.guess_type(filenamepath)

    if ctype is None or encoding is not None:
      # No guess could be made, or the file is encoded (compressed), so
      # use a generic bag-of-bits type.
      ctype = "application/octet-stream"

    maintype , subtype = ctype.split('/' , 1)

    if maintype == 'text':
      fp = open(filenamepath)
      attachment = MIMEText(fp.read() , subtype)
      fp.close()
    elif maintype == 'image':
      fp = open(filenamepath , 'rb')
      attachment = MIMEImage(fp.read() , subtype)
      fp.close()
    elif maintype == 'audio':
      fp = open(filenamepath , 'rb')
      attachment = MIMEAudio(fp.read() , subtype)
      fp.close()
    else:
      fp = open(filenamepath , 'rb')
      attachment = MIMEBase(maintype , subtype)
      attachment.set_payload(fp.read()) #Actual message
      fp.close()
      encoders.encode_base64(attachment) # Encode the payload using Base64

    return attachment

  def set_attachment_type(self,message):
    if(self.is_directory()):
      for filename in os.listdir(self.attachment_path):
        filenamepath = os.path.join(self.attachment_path , filename)
        attachment = self.guess_and_get_attachment_type(filenamepath)
        # Set the filename parameter
        attachment.add_header('Content-Disposition', 'attachment', filename = filenamepath)
        message.attach(attachment)

    elif(self.is_file()):
      attachment = self.guess_and_get_attachment_type(self.attachment_path)
      # Set the filename parameter
      attachment.add_header('Content-Disposition', 'attachment', filename = self.attachment_path)
      message.attach(attachment)
    else:
      print("Unable to open file or directory")

    return message



def main():
  #Constants
  GMAIL_SERVER = "smtp.gmail.com:587"
  FROM_ADDRESS = "xyz@gmail.com"
  TO_ADDRESS = "xzy@gmail.com"

  auth = Security("xyz@gmail.com" , "MySuperSecretPassword")
  attachment = Attachment("/path/to/attachment/file/or/directory")
  email = Email(FROM_ADDRESS ,TO_ADDRESS, "Hi from class Python" , "OOPs Python at Work!!" ,attachment,GMAIL_SERVER )

  email.send_message(auth)
if __name__ == '__main__':
  main()