What I do, when I download .rar files, I download them, I let mac append a .txt filename to it, and when I'm finished downloading all my .rar files, I select them all and I open workflow (by right clicking or Control+Left click - Automator - Create Workflow) and I choose the option rename finder items, I choose replace text, and I replace the ".rar.txt" for ".rar" that works fine for me, more work, but eh, Mac programs can't be perfect! (It's still better than windows, that's one thing! lol)
Hope that helps you out! But I wish there was an option on Safari to not append a .txt filename to certain files, that would make my downloading much easier, and it would make it more enjoyable! But eh, maybe Apple will soon 'fix' that 'bug' on Safari, who knows?