Notes about the THNL file extension:
To open this file type, a required software must be installed. Without it you only see a Windows message "The following file cannot be opened
" (Windows 7) or "How do you want to open this file?
" (Windows 10) if you double-click the file. There click on "More apps"
and choose a suitable software to view such files on your PC or from email attachments.
The THNL file type is primarily associated with 'Graphics Workshop for Windows Thumbnail'.
Detailed information for file extension THNL:
Primary association: Graphics Workshop for Windows Thumbnail
To change file associations press Windows Key
and then Apps and Features
> Default apps
> Choose default apps by file type
Invalid file associations could be the result of underlying issues within your Windows PC.