Adding git lfs to pre-existing repo

Say that you already have .mp4 files committed to existing repository, but would like to change mind, and you would like to add mp4 file in LFS, here's how you do it.

git rm --cached *.mp4
git add *.mp4
git commit -m "convert all mp4 files to lfs"

To check if mp4 files are added, you can run below command to check:

git lfs ls-files

Reference

More information can be found here: Git LFS Tutorial