Allegro is a game programming library. The PACKFILE format is a format containing the game's assets in a way which allegro can read (normally: bitmaps, run-length-encoded sprites, Amiga Module music files from libdumb).
The magic for this file format is
slh! (compressed) or
The allegro library is open source and comes with utility libraries for editing PACKFILEs.
Version 5 of Allegro replaces PACKFILE with standard zip utilities.