Slightly off-topic, but forcing me to download an image preview when I could quite as easily view it in my browser drives me insane, now I have a file I don't want that I didn't expect and have to delete it! Silly.
That's a browser issue. My browser asks if I want to download, or if I want to view internally (and if so, what mime type; to which I replied image/png).
Don't blame your poor configuration on the author of web pages. Ultimately clients should control the web experience, not servers.