`sniff` searched only the leading 4096 bytes for the content-types part, but
OOXML writers may place that part anywhere in the archive. Recent PowerPoint
builds (16.x) store it as the last zip entry, so every deck saved by them was
rejected with "not a presentation DeckCraft can read" although it imports fine.
In one real folder 169 of 300 decks were affected.
Look the part up in the zip's central directory instead. The regression test
re-packs the sample deck with the part last behind 64 KB of padding and
checks that it is both recognised and imported.
Co-authored-by: Matthias Weber <webermatt@me.com>
Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>
