What is "ilbm"

The LBM file extension is an image file in Interchange File Format (IFF) structure to store image or palette data. Most commonly you can find these files in InterLeaved BitMap (ILBM) format, but some games are using a similar, but simpler-to-use Planar BitMap (PBM) format. Both formats commonly exist under the .lbm file extension, occasionally appearing also as .bbm extension.

ILBM and PBM formats are used by games from late 80s and early 90s that either originated from Amiga platform or had their assets such as graphics done on Amiga.