Download function during sideload will always add double extension: .jpg.jpg

If `$desired_filename` is not set, then imported image will have duplicated extension: `.jpg.jpg`, `.png.png`, etc.

Related commits:

* https://github.com/Automattic/newspack-migration-tools/pull/115/commits/1fe05faee754d70a70d4ddb68084dc7dd2c83ec0 (check was added)
* https://github.com/Automattic/newspack-migration-tools/pull/115/commits/6c92a2b09b50570e20638d201f6630516f2ef488 (desired filename passed to function)

The new check always be false: `array_search( pathinfo( $path, PATHINFO_EXTENSION ), wp_get_mime_types() )`.

Based on how `wp_get_mime_types` array is structured, the check should probably be one of the following:

* Array key exists: `array_key_exists( pathinfo( $path, PATHINFO_EXTENSION ), wp_get_mime_types() )` - but this doesn't match on `jpg|jpeg|jpe`.

* Mime match: `array_search( mime_content_type( $tmpfname ), wp_get_mime_types() )`

I'm not quite sure why that check is there...maybe to make sure the file extension matches the mime and if not, add the correct extension?

For now, setting a `$desired_filename` before calling the import will bypass the check.