Hi 

You can create those files as ir. attachment records and  update it to the attachment_ids of the mail template 

Try this example

data_record = base64.b64encode(file)
ir_values = {
    'name': 'Name of the Record',
    'type': 'binary',
    'datas': data_record,
    'store_fname': data_record,
    'mimetype': 'image/jpeg',
    'res_model': 'product.product'
}
attachment = self.env['ir.attachment'].sudo().create(ir_values)
email_template = self.env.ref('module_name.mail_template', raise_if_not_found=False)
email_template.attachment_ids = [(4, attachment.id)]
email_template.sudo().send_mail(self.id)
email_template.attachment_ids = [(5, 0, 0)]

Attach a file in email template

Regards