correct it doesnt open straight from the file only by opening through sinfig itself.
We cannot find any file called project.sifz the files are made comepletely on there own not from any other synfig files.
Reported here: [Windows] Synfig doesn't open file in a file server via double-click · Issue #2212 · synfig/synfig · GitHub
Please check if my description is correct.