Some EPUB writers (e.g., "ebookredo") emit a non-standard local file
header signature on the first entry — bytes like PK\x03\x02 instead of
PK\x03\x04. The archive is still readable via the End of Central
Directory record, and @zip.js/zip.js handles it without complaint, but
DocumentLoader.isZip() rejected the file at the magic-bytes gate before
zip.js ever ran. The user saw "Unsupported or corrupted book file" on
a perfectly readable EPUB.
Drop the strict 4th-byte equality check. PK\x03 alone identifies a
local file header — no other ZIP record signature starts that way — so
loosening the check is safe and aligns with what tolerant ZIP parsers
already do.
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Huang Xin
c30a59a9ed