diff --git a/lib/middleware/karma.js b/lib/middleware/karma.js
index 4e27d4a52..eb86b5ef8 100644
--- a/lib/middleware/karma.js
+++ b/lib/middleware/karma.js
@@ -58,6 +58,12 @@ function getXUACompatibleUrl (url) {
   return ''
 }
 
+function reportFileTypeError (fileType, path) {
+  log.error(`Invalid file type (${fileType}) for ${path}.\n` +
+  '  Specify a valid type in the configuration file.\n' +
+  '  See http://karma-runner.github.io/latest/config/files.html')
+}
+
 function createKarmaMiddleware (
   filesPromise,
   serveStaticFile,
@@ -163,15 +169,8 @@ function createKarmaMiddleware (
             const fileType = file.type || file.detectType()
 
             if (!FILE_TYPES.includes(fileType)) {
-              if (file.type == null) {
-                log.warn(
-                  'Unable to determine file type from the file extension, defaulting to js.\n' +
-                  `  To silence the warning specify a valid type for ${file.originalPath} in the configuration file.\n` +
-                  '  See http://karma-runner.github.io/latest/config/files.html'
-                )
-              } else {
-                log.warn(`Invalid file type (${file.type || 'empty string'}), defaulting to js.`)
-              }
+              reportFileTypeError(fileType.length ? fileType : '<empty>', file.originalPath)
+              continue
             }
 
             if (!file.isUrl) {
diff --git a/test/e2e/helpful-logs.feature b/test/e2e/helpful-logs.feature
index 08d84b7b2..f80141985 100644
--- a/test/e2e/helpful-logs.feature
+++ b/test/e2e/helpful-logs.feature
@@ -18,7 +18,5 @@ Feature: Helpful warning and errors
     When I start Karma
     Then the stdout matches RegExp:
       """
-      WARN \[middleware:karma\]: Unable to determine file type from the file extension, defaulting to js.
-        To silence the warning specify a valid type for .+modules/minus.mjs in the configuration file.
-        See http://karma-runner.github.io/latest/config/files.html
+      ERROR \[middleware:karma\]: Invalid file type \(mjs\) for .+modules/minus.mjs\.
       """
diff --git a/test/unit/middleware/karma.spec.js b/test/unit/middleware/karma.spec.js
index 8a968262c..a11e16c1b 100644
--- a/test/unit/middleware/karma.spec.js
+++ b/test/unit/middleware/karma.spec.js
@@ -329,11 +329,11 @@ describe('middleware.karma', () => {
 
   it('should not change urls', (done) => {
     includedFiles([
-      new Url('http://some.url.com/whatever')
+      new Url('http://some.url.com/whatever.js')
     ])
 
     response.once('end', () => {
-      expect(response).to.beServedAs(200, 'CONTEXT\n<script type="text/javascript" src="http://some.url.com/whatever" crossorigin="anonymous"></script>')
+      expect(response).to.beServedAs(200, 'CONTEXT\n<script type="text/javascript" src="http://some.url.com/whatever.js" crossorigin="anonymous"></script>')
       done()
     })