Restore file format and icons to default - Windows 8.1, 8, 7, Vista

Steps:

- Open "regedit" from Run or Search bar.
- Find "HKEY_CURRENT_USER"
- Open "Software"
- Open "Microsoft"
- Open "Windows"
- Open "CurrentVersion"
- Open "Explorer"
- Open "FileExts" - Here is all the file extensions.
- Locate the file extension you wish to change back to default. This case find ".dll"
- Delete both folders "OpenWithList" and "UserChoice"

Press F5 to refresh the registry and then restart the computer.

Or you could just simply open Task Manager and close "explorer.exe" and start it again.