How to debug [middleware:karma]: Invalid file type, defaulting to js. statements.

[vort3xxx]

After upgrading to karma 3.1.0+ I see a new error.

[middleware:karma]: Invalid file type, defaulting to js. statements.

Unlike other issue posts with this issue, it is not apparent what file type it is seeing that offends karma. It is not actually echoing anything to the console like .svg that can be used to identify the problem.

Karma should tell me what specific files it is not happy about loading. Instead it gives me a generic error with insufficient contextual information to solve my problem.

Environment Details

• Karma version: All versions from 3.1.0 and up.

• Relevant part of your karma.config.js file

files: [
            { pattern: 'test/test-index.js' }
        ],

I'm using they "Alternate usage" with an index file.