text: import os
  import re
  
  from django.db import migrations, models
  from django.db.backends.postgresql.schema import DatabaseSchemaEditor
  from django.db.migrations.state import StateApps
  
  attachment_url_re = re.compile(r'[/\-]user[\-_]uploads[/\.-].*?(?=[ )]|\Z)')
  
  def attachment_url_to_path_id(attachment_url: str) -> str:
      path_id_raw = re.sub(r'[/\-]user[\-_]uploads[/\.-]', '', attachment_url)
      # Remove any extra '.' after file extension. These are probably added by the user
      return re.sub('[.]+$', '', path_id_raw, re.M)
  
  def check_and_create_attachments(apps: StateApps,
                                   schema_editor: DatabaseSchemaEditor) -> None:
      STREAM = 2
      Message = apps.get_model('zerver', 'Message')
      Attachment = apps.get_model('zerver', 'Attachment')
      Stream = apps.get_model('zerver', 'Stream')
      for message in Message.objects.filter(has_attachment=True, attachment=None):
          attachment_url_list = attachment_url_re.findall(message.content)
          for url in attachment_url_list:
              path_id = attachment_url_to_path_id(url)
              user_profile = message.sender
              is_message_realm_public = False
              if message.recipient.type == STREAM:
                  stream = Stream.objects.get(id=message.recipient.type_id)
                  is_message_realm_public = not stream.invite_only and stream.realm.domain != "mit.edu"
  
              if path_id is not None:
                  attachment = Attachment.objects.create(
                      file_name=os.path.basename(path_id), path_id=path_id, owner=user_profile,
                      realm=user_profile.realm, is_realm_public=is_message_realm_public)
                  attachment.messages.add(message)
  
  
  class Migration(migrations.Migration):
  
      dependencies = [
          ('zerver', '0040_realm_authentication_methods'),
      ]
  
      operations = [
          # The TextField change was originally in the next migration,
          # but because it fixes a problem that causes the RunPython
          # part of this migration to crash, we've copied it here.
          migrations.AlterField(
              model_name='attachment',
              name='file_name',
              field=models.TextField(db_index=True),
          ),
          migrations.RunPython(check_and_create_attachments, elidable=True),
      ]
  