ComfyUI shows tickmarks instead of JXL previews: cause and one-line patch
themofostarboi_v2 · reddit · 2026-10-19
- Problem: .jxl images saved via nodes like Save Image Extended write fine to disk, but ComfyUI's sidebar/history only shows a green tickmark instead of a thumbnail.
- Cause: Not a browser decoding issue — ComfyUI's frontend package has a hardcoded image-extension whitelist in formatUtil-.js (png/jpg/jpeg/gif/webp/bmp/avif/tif/tiff/svg) that omits jxl, so the file is classified as other, isPreviewableMediaType() returns false, and no thumbnail is rendered.
- Fix: Add jxl to both whitelists (the formatUtil array and the Set in main-.js). The author provides an automated Python patch script plus manual edit instructions.
- Apply: Restart ComfyUI and Ctrl+F5 in the browser; JXL thumbnails then render correctly.
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