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:
- Go to Control Panel > Programs and Features
- Click Turn Windows features on or off
- Expand World Wide Web Services > Common HTTP Features
- Tick Static Content
- 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.