Sends a download as a response.
Source
download(File file, {String filename}) {
header("Content-Disposition",
'Content-Disposition: attachment; filename="${filename ?? file.path}"');
header(HttpHeaders.CONTENT_TYPE, lookupMimeType(file.path));
header(HttpHeaders.CONTENT_LENGTH, file.lengthSync().toString());
responseData.add(file.readAsBytesSync());
}