The file format .wtbd is essentially a renamed zip file. This contains any images that were saved inside the program, allowing a file saved on one computer to be opened, and edited on another. Versions prior to 0.39.3 did not do this.
The zip also contains a "save.data" file - this contains various information about the program state. This includes:
This allows a user to carry on from where they were last editing their file.
When you load in .wtbd file, it may take a while to load as all the thumbnails need to be refreshed, which can take a few seconds; depending mainly on how many sheets are contained in the saved file.