Because of the issue described in T12611, changing the filetype of the Next wallpaper is
problematic and will cause a black screen on upgrade.
This terrible horrible patch works around the issue by using the .png filename extension
even though this is not accurate. That way, the file names are the same as the old
versions of the Next wallpaper and will be loaded correctly. Using the wrong filename
extension does not actually cause any problems, it's just inaccurate. a README file is
placed next to the images to explain this.
I'm sorry.
Long-term, we should investigate T12611.