If you are working with 2023.11 IDE, chances are that it has removed the file names from the extension upon import, as per linked bug report. You can check this by double-clicking the extension in the resource tree.
You could fix this manually by comparing the file contents to the ones in the demo project and assigning the correct name to each file in the extension.
Or, if you have moved to GM2024 IDE, you can remove the extension and use Add Existing (or Tools ➜ Create Local Package) to copy the extension from the demo project to yours.