CSS and JS files not being served correctly by IIS

If you receive a warning that that CSS and/or JS files are being ignored due to incorrect mime type. It could be IIS is not serving static content. Another indication is a blank content type field for static content in the IE developer tools network tab.

To resolve the issue:

  1. Go to Control Panel > Programs and Features
  2. Click Turn Windows features on or off
  3. Expand World Wide Web Services > Common HTTP Features
  4. Tick Static Content
  5. Click OK

Another cause is CSS and JS mime types being incorrect registered on the IIS server.  To check open regedit, expand HKEY_CLASSES_ROOT, find .css and check/change the Content Type value to text/css. Repeat for .js where the Content Type value should be text/plain.