in my backend .py 

attachment_ids = fields.Many2many('ir.attachment', string="Attachments")

in my xml file
<field name="attachment_ids" widget="many2many_binary"/>

in my website template.xml file
<input type="file" multiple="true" name="attachment_ids"
       data-show-upload="true" data-show-caption="true" lass="file"
       data-show-preview="true" id="attachment_ids.id"/>

in my controller.py
@http.route('/assignment/submited', type='http', auth='public', website=True)
def portal_submited_assignment_of_student(self, **kw):
    assignment_ids = request.env['op.assignment.sub.line'].sudo().create(kw)
    for assinement in assignment_ids:
        assinement.state = 'submit'
    values = {}
    if kw.get('attachment_ids', False):
        Attachments = request.env['ir.attachment']
        name = kw.get('attachment_ids').filename
        file = kw.get('attachment_ids')
        assignment_id = kw.get('assignment_id')
        attachment = file.read()
        attachment_id = Attachments.sudo().create({
            'name': name,
            'datas_fname': name,
            'res_name': name,
            'type': 'binary',
            'res_model': 'op.assignment.sub.line',
            'res_id': assignment_id,
            'datas': attachment.encode('base64'),
        })

    return request.render("portal_assignments.portal_submited_assignment_of_student",
                      {'assignment_ids': assignment_ids})