git archive -o something.tar.zst but file info just says "POSIX tar archive"

Hi,

I noticed the following (possible bug?) when I tried to create zstd tar archive (.tar.zst) with `git archive`.

First, I created the plain tar archive with `git archive`, then extract and rearchive to zstd tar achive:

``` (on the repo)

$ git archive -o /tmp/something.tar --prefix=something/ HEAD

(outside the repo, on /tmp)

$ tar xvf something.tar $ tar --zstd -c -v -f something.tar.zst something/ ```

I checked that the archive was indeed zstd tar archive:

``` $ file something.tar.zst something.tar.zst: Zstandard compressed data (v0.8+), Dictionary ID: None ```

Now I created the same archive with `git archive` directly:

``` (on the repo)

$ git archive -o /tmp/something1.tar.zst --prefix=something/ HEAD ```

But that archive info (with `file`) was something different:

``` (outside the repo, on /tmp) $ file something1.tar.zst something1.tar.zst: POSIX tar archive ```

I expected that `something1.tar.zst` was proper zstd tar archive, and not plain archive like above.