[File] [PATCH] magic/Magdir/typeset: detect Fabylon FAB2 book container (.fab)

[Martijn Benders]

Hello,

A small magic patch adding detection for the FAB2 book container, extension
.fab.
I am the developer of the format's specification, so the byte sequence
below is
authoritative rather than reverse-engineered.

The signature is a fixed 16-byte ASCII string at offset 0, with no preceding
bytes. The container generation is inside the magic string itself, which is
why
the patch matches the exact string rather than a prefix.

ASCII FABYLON_BOOK_02!
Hex 46 41 42 59 4C 4F 4E 5F 42 4F 4F 4B 5F 30 32 21

Placed in Magdir/typeset because the container stores typeset output —
resolved
line geometry rather than reflowable markup. Move it if you would rather it
lived
elsewhere.

--- typeset.orig
+++
https://www.google.com/url?q=http://typeset.new&source=gmail&ust=1787035263307000&sa=E
@@ -6,3 +6,7 @@
0 string Interpress/Xerox Xerox InterPress data
>16 string / (version
>>17 string >\0 %s)
+0 string FABYLON_BOOK_02! Fabylon FAB2 book container
+!:ext fab
+0 string FABYLON_REALM_01 Fabylon book container (legacy)
+!:ext fab

TESTED with file-5.48. The patched typeset file compiles clean with `file
-C -m`,
and:

real .fab file -> Fabylon FAB2 book container
FABYLON_REALM_01 header -> Fabylon book container (legacy)
FABYLON_BOOK_03! header -> no match (correct: an unknown future generation
must not be claimed as FAB2)
PDF and EPUB -> no match

Before the patch a .fab is reported as plain "data" /
"application/octet-stream".

A publicly downloadable test file, 10266 bytes:

https://www.google.com/url?q=https://github.com/mbenders/fab-format/blob/master/samples/fab-book-format-en.fab&source=gmail&ust=1787035263307000&sa=E

Specification:

https://www.google.com/url?q=https://fabylon.net/fab/specification/&source=gmail&ust=1787035263307000&sa=E

I have deliberately left out a `!:mime` line. A vendor-tree registration for
application/vnd.fabylon.book was submitted to IANA on 2026-08-17 and is not
yet
approved, and I did not want to put an unregistered type in your database.
Once
IANA confirms I will send a one-line follow-up patch adding it, if you
would like
that.

The legacy FABYLON_REALM_01 entry covers an earlier generation with a
different
internal structure. Current software still reads it; nothing writes it any
more.
Drop that hunk if you would rather not carry a retired generation.

Thanks for file(1) — it has been the first thing I reach for on an unknown
file
for twenty years.

Kind regards,

M.H.H. Benders
Fabylon, Karjalohja, Finland
m.benders at fabylon.net
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