Extracting files from partial downloads on Mac
Solution 1:
You can't just change the extension on a .download 'file' & expect it to be seen as anything else, because it's not a file, it's a folder - or strictly, Package, like an app or your Photos library etc.
If you right click it & select "Show Package Contents" then you can see inside it - though if it'a a partial file there may simply be nothing to see. tbh, I don't know how it handles the contents internally, but they're not even visible with 'show hidden' switched on.
If you double-click it, or right click, Open, it will attempt to complete the download.