Every example that I've seen for paperclip shows it being used with images, not with arbitrary files. Well, it works just fine with arbitrary files -- as long as the name of the model which houses your files is not "Attachment". That name somehow collides with "Paperclip:Attachment".
Something like "Medium" or "MyAttachment"will work fine. I'd be interested to know if "File" works, but I didn't try it for myself.