This is the first time I've tried to submit code to the MediaWiki community, so please accept my apologies if I have gone about doing so the wrong way. (and apologies the extension itself is such a hack and/or is already redundant because of others' work I wasn't aware of - I've noted SpecialUploadLocal recently)
SpecialMultipleUplaoadViaZip has been added as a meta.wikimedia.org page including the special page extension's source code. As that page describes, this special page provides a form for uploading a ZIP file that is subsequently decompressed and passed directly to a class extended from the existing MediaWiki file upload form.
I needed a multiple file uploading tool for a local intranet wiki I manage(ish). I suspect this feature might have already been scheduled to be included for future releases in a manner far cleaner than the hack I've done. Still, I figured it still might be useful for the community even so (e.g. possibly for those running to-be-legacy MediaWiki releases into which this extension might be able to be installed).
Best wishes, Dave