When you save a mashup project from the Mashup editor, a QEXT file is automatically included in the project. A QEXT file is a JSON file, and contains all metadata required by the Mashup editor to recognize the mashup project. It is also required if you want to launch your mashup from Qlik Sense Desktop.
The QEXT file is automatically created by the Mashup editor. You need to manually add a QEXT file if you have created a mashup project not using the Mashup editor, and you want it to be accessible from the Mashup editor.
The QEXT file extension must be in lower case letters, for example MyMashup.qext.
The Workbench load file (WBL) is used by the Mashup editor, and lists the names of all files included in the mashup project. When you save a mashup project from the Mashup editor, a WBL file is automatically included in the project.
Example: WBL file content
MyMashup.qext; MyMashup.html; MyMashup.js
I was under the impression WBL file was required to import an extension