scanner: use a blacklist instead of a whitelist

scanner: use a blacklist instead of a whitelist

This is much closer to what we did before with mimetypes. Using a whitelist
turns out to be a bad idea since repositories seem to be randomly filled with
executable images and documents, which trigger the scanner.

In an ideal world the scanner would complain about all of those. For now, just
warn about the possibility of them being hidden binaries.