apache/answer · warning

unsupported image format: %s

Error message

unsupported image format: %s

What it means

formatSpecificConfigCheck decodes an image's header to enforce a max-megapixel limit, but only supports jpeg, png, and gif. If the file extension is anything else, it returns this error before attempting a decode. It is an explicit allowlist failure, not a decode error.

Source

Thrown at pkg/checker/file_type.go:101

	return true
}

// formatSpecificConfigCheck decodes image config using a format-specific decoder
// based on the file extension. This avoids calling image.DecodeConfig() which
// dispatches by magic bytes and can invoke unintended decoders (e.g., TIFF)
// registered by transitive dependencies.
func formatSpecificConfigCheck(file io.Reader, ext string, maxImageMegapixel int) error {
	var config image.Config
	var err error
	switch ext {
	case "jpg", "jpeg":
		config, err = jpeg.DecodeConfig(file)
	case "png":
		config, err = png.DecodeConfig(file)
	case "gif":
		config, err = gif.DecodeConfig(file)
	default:
		return fmt.Errorf("unsupported image format: %s", ext)
	}
	if err != nil {
		return fmt.Errorf("decode image config error: %v", err)
	}
	if imageSizeTooLarge(config, maxImageMegapixel) {
		return fmt.Errorf("image size too large")
	}
	return nil
}

// formatSpecificImageCheck fully decodes the image using a format-specific decoder.
func formatSpecificImageCheck(file io.Reader, ext string, _ int) error {
	var err error
	switch ext {
	case "jpg", "jpeg":
		_, err = jpeg.Decode(file)
	case "png":
		_, err = png.Decode(file)

View on GitHub (pinned to 3b9f137061)

Solutions

1. Convert the image to a supported format (JPEG/PNG) before upload, or use one of the allowed formats.
2. If WebP support is needed, add a case "webp" using golang.org/x/image/webp's DecodeConfig.
3. Align the allowed-upload-format config with the formats this checker supports.
4. Reject early in the UI by filtering the file input accept attribute to supported types.

Example fix

// before
default:
    return fmt.Errorf("unsupported image format: %s", ext)
// after
import "golang.org/x/image/webp"

case "webp":
    config, err = webp.DecodeConfig(file)
default:
    return fmt.Errorf("unsupported image format: %s", ext)

var allowed = map[string]bool{"jpg": true, "jpeg": true, "png": true, "gif": true}
ext := strings.ToLower(strings.TrimPrefix(filepath.Ext(filename), "."))
if !allowed[ext] {
    return fmt.Errorf("format %q not supported; use jpg, png or gif", ext)
}

func isSupportedImageExt(name string) bool {
    switch strings.ToLower(filepath.Ext(name)) {
    case ".jpg", ".jpeg", ".png", ".gif":
        return true
    }
    return false
}

err := checker.CheckImage(file, maxMegapixel)
if err != nil {
    if strings.Contains(err.Error(), "unsupported image format") {
        return http.StatusUnsupportedMediaType, "only jpg, png and gif images are supported"
    }
    return http.StatusBadRequest, err.Error()
}

When it happens

Trigger: Uploading an image whose extension is not jpg/jpeg/png/gif (e.g. webp, bmp, svg, tiff) while it passed the generic file-type check.

Common situations: Modern uploads default to WebP from screenshots/CDN-converted files; users rename non-images to .png or upload HEIC from phones; site config allows a format the checker does not implement.

Related errors

• File validation failed
• validate check exception
• base.request_format_error
• error.password.space_invalid
• decode image config error: %v

AI-assisted analysis of apache/answer@3b9f137061 (2026-09-05). Data as JSON: /api/errors/fc7f058a32970d4f. Report an issue: GitHub.