please show me on how i can do it with the below code.

To restrict specific file extensions as attachment on the helpdesk.ticket model, you can override the create and write methods of this object. 

class HelpdeskTicket(models.Model):

    _inherit = 'helpdesk.ticket'

    @api.model

    def create(self, values):

        attachments = values.get('attachment_ids', [])

        for attachment in attachments:

            datas_fname = attachment[2].get('datas_fname', '')

            if datas_fname.endswith(('.exe', '.bat')):

                raise ValidationError('You cannot upload files with .exe or .bat extensions.')

        return super(HelpdeskTicket, self).create(values)

I've used '.exe' and '.bat' as an example, you can add the extensions you want and then in the same manner define a write method and I hope this should solve your problem.