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Even without the shopt -s extglob the folllowing is expanded: echo {*,*/*,*/*/*}.{opt,texi}
A shopt -s nullglob suppresses non-matching patterns.

grep -l prints the file names so you can process them with xargs.
grep --include="*.org" --include="*.texi" -lir "Abstract" . | xargs headBut xargs has a problem with space characters in file names....