Which archive formats are supported by systemd-sysupdate?

[Nils Kattenbeck <nilskemail@xxxxxxxxx>]

G'day,
I was looking into using sysupdate but did not find any information
about which archive formats are supported for "url-file". The man page
simply states that files are decompressed without specifying it any
further and uses .xz files in the examples.
Are other formats like .zstd also supported? Is there a list somewhere
which I can look at? I skimmed the source code but did not find
anything at a first glance (apart from systemd-pull which does not
seem to perform decompression).

Greetings, Nils