How about this:
=LET(
n,CELL("filename"),
f,SEARCH("[",n),
t,SEARCH("]",n),
p,MID(n,f+1,t-f-1),
SUBSTITUTE(p,".xlsx",""))
Old formula style:
=SUBSTITUTE(MID(CELL("filename"),SEARCH("[",CELL("filename"))+1,SEARCH("]",CELL("filename"))-SEARCH("[",CELL("filename"))-1),".xlsx","")
Andreas.