once you've seen how the form entries are transferred over HTTP connection, you'll understand it's impossible to have anything nested with the multiform request as well as with the url-encoded request.
things are very simple. multipart form request has the format of:
--- content-boundary ---
Content-Disposition: form-data; name="form_data_name"
[content (skipped)]
--- content-boundary ---
that's it. there's stream of single data form entries in the format: [form entry name] -> [form entry content] that repeats for every entry in the form. it's not recursive, therefore there may not be any nested structures.
Sources: