relatively pleased that i think i found the code section to get the "possible file id types" blob out
$ grep -r 'mlist' . | grep -v apprentice | grep -v .sh
./src/softmagic.c: struct mlist *ml;
./src/softmagic.c: for (ml = ms->mlist[0]->next; ml != ms->mlist[0]; ml = ml->next)
./src/softmagic.c: struct mlist ml;
./src/file.h:struct mlist {
./src/file.h: struct mlist *next, *prev;
./src/file.h: struct mlist *mlist[MAGIC_SETS]; /* list of regular entries */
./src/file.h:protected int file_magicfind(struct magic_set *, const char *, struct mlist *);