[FIX] website_form: link attachments to binary fields

If the name of a file input field is 'x_binary_input', before
commit 9be29371abcdfe57b1738484c5057ad81e76b00d (opw 2092653),
self.form_fields would have names of the form 'x_binary_input[i]'.
After this commit, they are of the form 'x_binary_input[i][j]'.
When calling extract_data, field_name would then be 'x_binary_input[i]'
instead of 'x_binary_input'. Therefore, the file would be considered as orphan
instead of being attached to x_binary_input.
We thus change the split so that it always returns 'x_binary_input'.

opw 2122624

closes odoo/odoo#41135

X-original-commit: 21ed2b971f66879fa3fa53dbd2c349ab39fa3fdb
Signed-off-by: fah-odoo <fah-odoo@users.noreply.github.com>

addons/website_form/controllers/main.py

@@ -122,7 +122,7 @@ class WebsiteForm(http.Controller):
             # If the value of the field if a file
             if hasattr(field_value, 'filename'):
                 # Undo file upload field name indexing
-                field_name = field_name.rsplit('[', 1)[0]
+                field_name = field_name.split('[', 1)[0]
 
                 # If it's an actual binary field, convert the input file
                 # If it's not, we'll use attachments instead