Add support for AVIF, JPEG XL, and HEIF image formats across the stack: - knotserver/xrpc: override MIME types for formats that Go's http.DetectContentType does not recognize (.avif, .jxl, .heic, .heif), using a switch statement like the existing .svg override - camo: add image/avif, image/heif, and image/jxl to the allowed MIME types list - appview/repo: add .avif, .jxl, .heic, .heif to the image extension list in the blob viewer Without these changes, AVIF files (and other modern formats) are rejected by the knot server with a 403 (detected as application/octet-stream), blocked by the camo proxy with a 415, and not previewed in the blob viewer. Signed-off-by: Niclas Overby <niclas@overby.me>