Ruby on Rails

What are you using to test? This is a bit off your topic as it sounds like you are writing controller specific tests. I used to do that, but in the last year I have started convering my ui and views heavily with testing and since doing this I find that I can achieve reasonably also testing the controller actions. I am sure some will argue against this but in the interest in time and quality I think I get more bang for my time covering how the user will use my ui. That said, I use rspec with capybara (steak) and using capybara, if there is a download link, I just click the link and then look at the page.body which gives me the content of the downloaded file.

I know you should be able to do so in a controller spec/test also, should be something like calling a get. I am not sure what your method is but if it has multiple formats then you would have to specify the format in a parameter I think, or do something like "get '/controller/action/file.pdf" or something like that. I would google for 'rails test file download', it looks like you will have some results you can at least piece together.