test: save WebP responses with the right extension

[MukundaKatta]

Summary

• fix the WebP MIME type match in daemon response handling
• save responses with a suffix instead of falling back to
• add a regression test for WebP image handling

Testing

• npm run build
• node --test --test-name-pattern="parsing" build/tests/daemon/client.test.js

Fixes #1898

[OrKoN]

thanks!