How to attach a newly created excel file to a post?

Question

Ruby on Rails

There is a mailer in which an excel file is generated and attached to the mailing list, everything is like here:
https://github.com/caxlsx/caxlsx_rails#mailers

xlsx = render_to_string layout: false, template: "my_dir/export"
xlsx_base64 = Base64.encode64(xlsx)
attachment = {mime_type: Mime[:xlsx], content: xlsx_base64, encoding: 'base64'}
attachments["file.xlsx"] = attachment

I need every file sent to be saved. To do this, I added a new model with has_one_attached:

class MyLog < ApplicationRecord
  has_one_attached :xlsx_file
end

And after creating the file, I want to create a new entry with this attached file:

MyLog.create(
  # xlsx - это то, что я выше создал. Возможно нужен xlsx_base64 или attachment
  xlsx_file: xlsx
)

The problem is that caxls creates a file and returns a string. This is according to their instructions for mailing attachments. For example, in the active_storage_attachment documentation, a file_field is used to get a file, and a file with the ActionDispatch::Http::UploadedFile class comes from it. If it is passed to xlsx_file, then everything works fine.

But I do not need to receive the file through this tag, but take the newly created one and attach it. How can this be done at all?