wp_handle_upload to override is_uploaded_file

[freediver]

wp_handle_upload currently does not support override for is_uploaded_file test.

This can be useful to handle dynamically generated uploads like for example when instead of hotlinking to a picture you want to download the picture given by the URL and handle it properly.

It would be nice to have everything under one roof, the function that can handle all user generated uploads.

[jacobsantos]

Use download_url() function instead.