setlocal EnableDelayedExpansion

set "input=%~n1"
set "ending=%~x1"
set "cleanin=!input:_original=!"
set "output=%cleanin%.mp4"

if /I "%ending%"==".mp4" (
  set "output=%cleanin%_remux.mp4"
)

ffmpeg -i %1 -c copy "%output%"

endlocal