I gave this a shot, if search for "tmp" in the actions, you'll see a few options to put a file in the tmp folder. However, I still run into an issue similar to yours where it's saying it can't find the file despite now using the tmp file path
Download a file looks like it puts it on Lindy's server, download to tmp looks like it puts it in pipedreams format
Hopefully that helps you along further until a lindy person can chime in
PS. Love your content!