Although it has nothing to do with this case, I will only introduce cases where many people do not have the wlt file in MacOS.
If you are using Safari on MacOS, the backed up wallet file will be named "Unknown". There is no extension (like .xxx), either. In addition, Windows Safari has not been tested.
In this case, changing the file name from "Unknown" to "Unknown.wlt" solves the problem.
However, since many people give instructions to search for files with a .wlt extension, it makes it harder to find problems.