Where to upload static html pages to one of my network sites?

I want to add a statically created manually uploaded html page to one of my network sites.

But I can’t figure out where to upload the .html file to…

The root directory of the main network site? Doesn’t work.
The blog.dir directory for the particular network site? Doesn’t work.
Creating a new ftp directory such that mainnetworksite.com/additionalsitedirectory/ ? Doesn’t work.

Where else is there to upload?

I.e. how can I get a .html page added to one of my network sites?

thanks mucho