What should a file tool show before you download?

I'm building KitForma and thinking about the moment between processing a file and clicking Download. A success message is useful, but it doesn't tell you whether the result is what you wanted.

For an image resize, I want to see the actual output dimensions and a preview. For a PDF merge, file order and a quick page check matter. For JSON, readable formatting and a syntax result answer different questions.

What is the smallest useful output check you look for in a browser tool? A concrete example of a confusing or reassuring result screen would help me decide what to improve. Please use a sample file rather than sharing private documents.