I'm amazed that this is normal behaviour in OSX. But if you name your folder "xxxyyzz.download", it creates a temporary download file. This is not the only problem I encountered (there are other extensions that give problems), but I was wondering if this could be solved? I have a naming preference of using dots in the folders - seems this is not a good idea on OSX?
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.
Or you could use a dash instead of a period. xxxyyzz-download