convert webp to jpg error: "no decode delegate for this image format" and "missing an image filename"

Google already provided the tool to decode webp images in the libwebp package, your uploaded file works on Arch.

dwebp file.webp -o abc.png

For the encoding tool, check the cwebp command.

In Ubuntu you can install the tools with:

sudo apt install webp

On RHEL/CentOS:

 yum install libwebp libwebp-tools

And you might consider using this online tool.

ffmpeg can do this. Useful if you already have ffmpeg. No need for installing other tools.

Simply:

ffmpeg -i file.webp out.png

Convert all webp files within a directory

find ./ -name "*.webp" -exec dwebp {} -o {}.png \;

Note: dwebp is in the libwebp package