[FIX] website_form: multiple input files with same name

By default, the name of a file input field is 'Custom File Upload'.
If there are two fields with the same name, then they both end up in
self.form_fields with the name 'Custom File Upload[0]', and their values
are concatenated, which are file objects. So the concatenation of two
files is a String, ('[object File],[object File]'), because JavaScript.
(And if you don't like it you don't like the web nor human progress.)

The resulting bug is that instead of adding attachments to the created
record, it adds the string message to the notes.

Adding the outer loop index disambiguates the names, so that all
attachments are created as intended.

opw 2092653

closes odoo/odoo#39669

X-original-commit: 5df3f662
Signed-off-by: Nans Lefebvre (len) <len@odoo.com>