[File] Regression introduced by nuget detection

[Christoph Biedl]

Christoph Biedl wrote...

> This looks like an unrelated change, none of the very few nuget files
> I've checked contain a .png or .md file. At the same time however, I
> reckon this will detect all (some?) .zip archives containing such a file
> as Microsoft OOXML.

Upon closer checking, this mis-detection seems to happen only under some
rare circumstances.

    Christoph