There are two types of extensions. Depending on what of extension you're creating, the folder structure may differ. The following files are always present, regardless of what type of extension you're creating:

• params.json
• splash.html
• splash.jpg
• thumbnail.jpg

The purpose of this file is to tell Mobirise what the name of the extension is, the description, the version of the extension and what logo to use. If you want to know more about this file, check out the params.json page.

This file contains the contents of what a user sees when the extension has been installed succesfully.

In most cases, the splash.html file contains an img-tag that points to an image. This image is displayed when the extension has been installed succesfully. The name of said image does not have to be splash.jpg, but it's very common to do so. It may as well be named extension.png. As you can see, it may as well be a PNG file.

Just like the splash.jpg file, the thumbnail.jpg does not have to be named this way. You can call it whatever you like, as long as you're referring to this file through the params.json file.