This proposal is not currently maintained and has been archived. At the time of archival, work to document and improve the 'webkitdirectory' behavior was ongoing in the File and Directory Entries API proposal.
The directory upload proposal can be found here: https://wicg.github.io/directory-upload/proposal.html.
A demo of the API using a polyfill (works only in Chrome) can be found here: https://wicg.github.io/directory-upload/index.html.