deus@ubuntu:~/ffmpeg.work$ cat test.sh
#!/bin/bash

inputfiles=(*.mp4)
outputfiles=("${inputfiles[@]/%.*/.test.mp4}")

echo "Input Files"

for item in "${inputfiles[@]}"
do
  printf "  %s\n" "$item"
done

echo "Output Files"
for item in "${outputfiles[@]}"
do
  printf "  %s\n" "$item"
done

deus@ubuntu:~/ffmpeg.work$ ./test.sh
Input Files
  test orig.mp4
  test.orig.mp4
Output Files
  test orig.test.mp4
  test.test.mp4