You must follow the requirements for specifying documents to cache:
Each item in the /rules property includes a glob pattern and a type:
The glob pattern is used to match the path of the document.
The type indicates whether to cache the documents that match the glob pattern.
The value can be either allow (to cache the document) or
deny (to always render the document).
It looks like you're trying to use the syntax for filter rules in the cache section. I think you'll need to use a glob pattern that matches the URL and extension using wildcards, so you may need to use multiple rules.