Chrome Extensions (CRX) are ZIP-files with an added header in the form of magic number + version number + public key length + signature length + public key + signature[1].
This small web app uses the File-api's to calculate the above header-length, skip it, and download it to your computer as a ZIP-file which can then be easily extracted.
You can try it for yourself at https://johankj.github.io/convert-crx-to-zip/.
MIT