mkdir summation
del summation\summation.txt
for %%f in (*.z80) do type "%%f" >> summation\summation.txt
del sum.txt
FOR /F "delims=`" %A IN ('dir /b /s ^| findstr ".ext1 .ext2 .ext3"') DO type "%A" >> sum.txt
Replace .ext1, .ext2, etc. with all the extensions you want to search for, eg ".z80 .asm .inc"