Naming a folder xxyyzz.download creates safari file?
You can remove the bundle document type for the .download extension.

You'll have to edit /Applications/Safari.app/Contents/Info.plist and remove the entry for the download extension under Document Types. The only disadvantage to this is that, whenever Safari updates, you'll have to do this again as your changes will be overridden.

I'd also make sure you make a backup of the Info.plist file first, before you make changes, incase you move the wrong line(s) or Safari gives you any issues after making the changes.