What is the TNL file format?
Files with the .tnl extension store image thumbnails in the Thumbnail Image format. Thumbnails are image miniatures that hold preview images, and .tnl files are generated automatically by image viewers and managers. Alternatively, .tnl files can be called image previews - displayed when browsing JPEG files, for example, to prevent opening large image files each time an image catalog or folder is accessed.
The .tnl format is used by XnView to quickly generate image previews. Thumbnail files are created upon the first time a given image folder is opened, and it may take some time for the process to complete and display generated miniatures. It is worth noting that .tnl files do not store full images, only thumbnails, and cannot be used to recover deleted or corrupted source images.
The .tnl extension is not a single standardized format - it is shared by at least three unrelated applications. GoPro Studio also used .tnl files as cover thumbnails for .mp4 video files, though GoPro Studio itself was discontinued in 2017. Nintendo's Super Mario Maker for Wii U stored course preview thumbnails with a proprietary header in .tnl files. In all cases the underlying data is typically JPEG-based; renaming a .tnl file to .jpg is a practical first step when attempting to open it in a standard image viewer.
Security & safety
RISK: LOWThumbnail image files; no executable code. Safe to open and safe to delete (they are cache/companion files).
Format details
in a nutshell- GoPro Studio Thumbnail - GoPro Studio (discontinued 2017) stored MP4 video cover thumbnails as .tnl JPEG files alongside the video.
- Super Mario Maker Course Thumbnail - Super Mario Maker (Nintendo Wii U, 2015) stored course preview thumbnails as .tnl files with a proprietary Nintendo header.
Programs that open TNL files
Windows3 apps
Any image viewer (after rename)
Free For XnView or GoPro TNL files: rename to .jpg and open in IrfanView, Windows Photos, or any JPEG viewer. 
Linux1 app
Technical details
deep spec| Encoding | Binary (JPEG-based in most implementations) |
| Underlying format | Typically JPEG (`FF D8 FF` header); Nintendo Wii U variant uses a proprietary binary header |
| Compression | JPEG DCT lossy compression (typical) |
| Color depth | 24 bpp RGB (standard JPEG thumbnail) |
| Typical dimensions | 160×120 to 1280×720 px (thumbnail use cases) |
| File size | 1 KB - 500 KB (small by design as thumbnail data) |
| Standardization | None - shared extension with no universal specification across applications |
| MIME type | `image/jpeg` (typical) |
| XnView usage | Cache file generated automatically on first folder open; stores image preview data for fast browsing |
| GoPro Studio usage | Cover thumbnail for `.mp4` video files; GoPro Studio was discontinued in 2017 |
| Nintendo Wii U usage | Super Mario Maker course preview thumbnail stored with a proprietary binary header |
| Recovery limitation | Cannot be used to recover deleted or corrupted source images; contains only the thumbnail, not the original |
| Released | early 2000s (XnView context); 2015 (GoPro Studio); 2015 (Super Mario Maker) |
TNL conversions
Community Q&A
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