Please see File | Settings (Preferences on Mac) | Editor | File Types.
Look for your file name mapped to the Text type or to the Auto-detect file type by content type.
Remove the incorrect mapping and it will fix the issue.
If you still can’t find the wrong mapping in the IDE settings, locate the options/filetypes.xml file in the Configuration directory. Close the IDE and either edit this file to remove the incorrect mapping or delete this file to reset all the file types to the defaults.
- You can also use Revert File Type Override / Override File Type file context menu actions.