The software was probably doing something that no Windows user would expect: when saving a map from Wilbur, you need to select the file type from the dropdown every time. Just entering the name or just clicking Save will get you the default type, which is PNG Surface. I am aware that it's not a great (or even good) design, but there are multiple types with the same extension (e.g. PNG Surface and PNG texture). If you open the file in Notepad and look at the first few bytes, an MDR file will read "WLBR" and the first few bytes of a PNG will read "‰PNG".