Use filter plugins when accessing files directly with the web interface

[msm-code]

Hey MSM! I think that filter-plugin can\should also be called when clicking the Download button (https://github.com/CERT-Polska/mquery/blob/fecbef082bcb4355abdab318b90ef0a6dc1b9330/src/app.py#L205), what do you think? So if my S3 bucket contains encrypted files, it is all nice when I can decrypt it prior of scanning (and indexing), but it's a problem when user downloads it encrypted

This observation is definitely correct, and the behaviour should be fixed.