Gmail is on the Google servers. Did you pay for Gmail? Did Gmail has access to your computer by default? So on.. What analogy are you making? When Gmail advertised "What is on your Gmail, stays on your Gmail"? :)

No, no analogy. GMail scans every email and every attachment. Server side because - of course - it’s an online service. It’s main feature is analyzing and indexing so it’s a direct example of a killer feature based on content analysis. Even spam filters are.

The same slippery slope applies there.