>>58712
quick fix for cases where the input file ending is also .mp4:

set "input=%~n1"
set "ending=%~x1"
set "output=%~dp1%~n1.mp4"

if /I "%ending%"==".mp4" (
  set "output=%~dp1%~n1_remux.mp4"
)

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