1. the css content was process with preprocessor, such as karma-less-preprocessor
2. karma/lib/preprocessor takes '.js' as suffix of file.contentPath in line 63.
3. karma/lib/middleware/common line 39, mime.lookup with file.contentPath
4. as the result of above, a requestUrl like /base/css/a.css, the content-type will response as 'application/javascript'

i'll send a pull request to fix it.