MODE Z exception for already compressed file types

[pinkduck]

It would be good if compression via MODE Z could be skipped when enabled for certain file types, known to already be compressed and thus not offering size reduction but time delay if compression goes ahead as currently.

Example file types: .rar, .rar5, .zip, .mp3, .mkv, .mp4, .jpg, .png, .jar

Also ideal if such exempted extensions could be maintained by user, perhaps per-site or globally.