How to use URLDownload if the url doesn't have the correct filetype extension?

Question

I'm using the new (in V11) function UrlDownload[]

I have a url I'd like to save to a local file, but the url is missing a (or has the incorrect) filetype extension. So I don't know how to construct the filename properly. Here's an example:

url = "http://t0.gstatic.com/images?q=tbn:ANd9GcRf1EqE2kyW12HSb9gZZ8eTIPqNgVkjFis4GkTTYONIpoQtkIde4zybZ4iAqGlIHQ_pnEX499Oa";
Import@url
URLDownload[url, "~/Downloads/" <> ToString@RandomInteger[1000000]]

I'd like to save it with the proper extension though. Any thoughts?

Also, URLDownload doesn't seem to be multithreaded, this is waaay too slow:

Answer 1

If I was downloading different types of images and I couldn't find out what their file types are a priori I'd do something like this perhaps:

url = "http://t0.gstatic.com/images?q=tbn:\
ANd9GcRf1EqE2kyW12HSb9gZZ8eTIPqNgVkjFis4GkTTYONIpoQtkIde4zybZ4iAqGlIHQ\
_pnEX499Oa";

callback[file_][_, "data", _] := (
  If[FileFormat[file] == "JPEG", CopyFile[file, "path/to/new/image.jpg"]];
  DeleteFile[file];
  )

file = CreateFile[];
URLSaveAsynchronous[url, file, callback[file]];

URLSaveAsynchronous is an asynchronous operation as the name suggests, which means that you can download many files simultaneously. I use CreateFile to create an empty file in my operating system's directory for temporary files and pass a custom callback function, using that file, to URLSaveAsynchronous. When URLSaveAsynchronous has downloaded the file it calls the callback function, which subsequently detects the file format using FileFormat, copies the file to wherever it's supposed to go, with the right file extension, and then deletes the file in the temporary directory.