[Fix]Mass_mailing module in put an attached file with the mail template,...

... and it was  sent with the mail to the recipients.

- Task: https://www.odoo.com/web#id=9833&view_type=form&model=project.task&menu_id=3942&action=327
- Pad: https://pad.odoo.com/p/r.rYaZxxlRhq1AGlP8