bmp/tiff/gif are decode-only — metadata().format may report them but there are no .bmp()/.tiff()/.gif() encoder methods. tiff decode rejects with error.code === "ERR_IMAGE_FORMAT_UNSUPPORTED" on Linux; gif decodes the first frame everywhere.
type
Image.Format
type Format = 'jpeg' | 'png' | 'webp' | 'heic' | 'avif' | 'bmp' | 'tiff' | 'gif'