A while back newgrounds started doing some weird shit with files and now flashes are all in a weird .zip format that fucks with every downloader around, not even youtube-dl works anymore. TL;DR is there any way to download these files and make them back into flashes?
Yes, right-click the web page that shows the video version and select "view page source". CTRL+F ".swf" and copy the URL to the flash file on uploads.ungrounded.net. Paste the URL in the Firefox address bar and hit enter (Firefox understands that "\/" is supposed to be "/"), finally press CTRL+S to save the flash.
If you are logged into a Newgrounds account you're given the option to watch the original flash instead of the video version when available. Why they require you to be logged in for this option I don't know.
Last time I tried this was like 2 years ago so hopefully you can still access all original flash files this way. There's really no reason for them to ever remove the original flash versions since they are less than 1% of the file size of the combined file size of the video versions they offer (1080p + 720p + 360p) so they SHOULD stay up forever but who knows how their brains are wired.