Wilbur generally only saves the heightmap in various formats. I never got around to implementing a full document type for it.
However, you can save various parts of the configuration to Lighting Definition (*.lgt), Brush Definition (*.brs), and Surface Definition (*.srf) files (those would be approximately the things on the Shader Setup dialog for the Wilbur shader, the Edit Paintbrush Settings dialog for painting operations, and the Height Field Computation dialog under Filter>>Calculate Height Field).
The MDR Surface file type will save a precise image of the floating-point heightmap in memory, including the information in the Map Information dialog under Surface>>Map Info. Note that the information that it asks you about for MDR files when saving was in the original concept but was never implemented.