… gimp_file_create_thumbnail().
One more case where "save" is misleading, and even more as it's not a
procedure where we control where an image is stored. We only say
basically "make a thumbnail which maps to this file according to my
platform's rules".
As a side fix, I also improved a bit the logic so that it allows @file
to be the exported or — as last fallback — the imported file.
And finally improve the function's docs.
6dfc9f96c5