In case someone is interested I made some progress in specifying the Diablo 1 archive format:
https://github.com/savagesteel/d1-file- ... /DIRBIN.md

Hi,

I'm trying to figure out the Diablo 1 archive format and also the graphics included in the archives.

The archives come as .DIR + .BIN files (e.g. LUMP.DIR and LUMP.BIN).
The .DIR file contains file names, the file offsets in .BIN file and the file size.
I'm still missing some details, but ...