Here's the most relevant section I could find from the original source:

"Chrome extensions can expose internal files to web pages through the web_accessible_resources field in their manifest.json. When an extension is installed and has exposed a resource, a fetch() request to chrome-extension://{id}/{file} will succeed. When the extension is not installed, Chrome blocks the request and the promise rejects.

LinkedIn tests every extension in the list this way."

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thayne an hour ago | parent [-]

It seems like it shouldn't let code originating from the site (as opposed to from the extension) to access that.

	▲	fractaled 15 minutes ago \| parent [-]
		I'm not sure you'd need to directly fetch to determine if they resolve. One could probably inject an img tag and see if it resolves.