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We answered your question on Facebook, and just saw this post here. It’s better to keep the discussion in one place ;). Anyway, I’ll copy the answer here for reference:
We redirect to the file itself. The attachment post normally has thin content, and is usually not good for SEO. Besides, an image can be used in multiple posts, or has no parent posts (if it’s uploaded to the library directly).
FYI: This is the default behavior in new WP installs.