@johnjbarton, Thank you for taking a stab at this. I pulled down this change today and it still results in output that does not help the issue.

After this change, the new command line output appears like this:

WARN [middleware:karma]: Invalid file type (), defaulting to js.

Given the new output, I still cannot ascertain which file is actually offending Karma. It's a little more readable, as I assume it's some extension-less file.

You must have a file in your config that has no filetype extension.