Thank you Jainesh,

I couldn't reply to your answer because of this stupid Karma. So your answer was helpful and inspired me to find the solution, but it will not work on Python 3, the following code is worked on Python 3:

from io import BytesIO
import base64
import xlsxwriter

        output = BytesIO()
        file_name = "%s.xlsx” % your_file_name
        workbook = xlsxwriter.Workbook(output)

        //your code here

        self.env['ir.attachment'].create({
            'name': file_name,
            'type': 'binary',
            'datas': datas,
            'store_fname': file_name,
            'res_model': model_name,
            'res_id': res_id,
            'mimetype': 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet'
        })