git rm --cached <filePath> does not unstage a file, it actually stages the removal of the file(s) from the repo (assuming it was already committed before) but leaves the file in your working tree (leaving you with an untracked file).
git reset -- <filePath> will unstage any staged changes for the given file(s).
That said, if you used
git rm --cached on a new file that is staged, it would basically look like you had just unstaged it since it had never been committed before.
Update git 2.24
In this newer version of git you can use
git restore --staged instead of
git reset. See git docs.