Hello, this is a Windows's long time issue becsuse it usually hide extensions of file names. File will be saved correctly when:
- you will enable "show file extensions" in the Folder Settings in the control panel
- you will manually append ".lvl" or ".lvlx" info file name, and you shouldn't see this problem.
On Mac OS and on Linux this issue is absence, file extensions is a part of filename and always visible.
Try, for example, to save any picture via mspaint for example, and you will see same issue.