On Windows, slicerExtensionWizard creates invalid icon files for extensions and modules. For example, the icon file for a default extension is only 6 bytes:

$ ls -al Slicer/Utilities/Templates/Extensions/Default/TemplateKey.png
-rw-r--r-- 1 max.smol Administ 17838 Jul 31 16:02 Slicer/Utilities/Templates/Extensions/Default/TemplateKey.png

$ ./S4D/Slicer-build/bin/slicerExtensionWizard --create MyExtension
Qt: Untested Windows version 6.2 detected!
copy template 'C:\dev\Slicer\Utilities\Templates\Extensions\Default' to
'c:\dev\MyExtension', replacing 'TemplateKey' -> 'MyExtension'
creating 'c:\dev\MyExtension\CMakeLists.txt'
creating 'c:\dev\MyExtension\MyExtension.png'
created extension 'MyExtension'

$ ls -la MyExtension
total 4
drwxr-xr-x 4 max.smol Administ 0 Aug 4 17:21 .
drwxr-xr-x 1 max.smol Administ 4096 Aug 4 17:21 ..
-rw-r--r-- 1 max.smol Administ 1041 Aug 4 17:21 CMakeLists.txt
-rw-r--r-- 1 max.smol Administ 6 Aug 4 17:21 MyExtension.png

This happens because the script treats all files as text when opening them to replace the template key. One fix is to open and write the files as binary.