Image sideloading update

Don't try to sideload file without a file extension, leads to WP upload
error, then "PHP Notice:  Undefined offset: 0 in
/Users/epsi/Sites/wptrunk/wp-admin/includes/media.php on line 811”.

Don’t abort and delete post on sideloading error(s), just return the
original content still linking to / using the original image.