How to upload file in website page with a normal HTML form ? 

On the template:

<template id="upload">
   <form action="process" method="POST" enctype="multipart/form-data" >
   Upload file:
   <input type="file" name="attachment"/> 
   <button string="upload" type="submit" name="submit">Upload</button> 
</form> 
</template>

On the controller

# -*- coding: utf-8 -*-
from odoo import http
from odoo.http import request
import base64

class VitUpload(http.Controller):

    @http.route('/vit_upload/upload', auth='public')
    def upload(self, **kw):
        return http.request.render('vit_upload.upload', {})

    @http.route('/vit_upload/process', auth='public', method="POST", csrf=False)
    def process(self, **post):
        if post.get('attachment', False):
            name = post.get('attachment').filename
            file = post.get('attachment')
            attachment = file.read()
            encoded = base64.b64encode(attachment)
            print(encoded)

        return "upload process success"

Main things:

The html form must have enctype="multipart/form-data"

The processing handler must encode the uploaded file content to base64 encoded string, then save the string to attachment or a field with type binary on a model.

Example:

payment = request.env['account.payment'].browse(payment_id) 
payment.attachment = encoded