Introduced in [22783] and [23094].
In 3.4, attachment_fields_to_save filter was only called in media_upload_form_handler() for a post array received from get_post(), which contained a guid field: tags/3.6/wp-admin/includes/media.php#517.
Since 3.5, it's also called in edit_post() for an array that contains an attachment_url field instead: tags/3.6/wp-admin/includes/post.php#L264.
I'd suggest checking both fields, as long as we still have both scenarios in core. Although the former one is no longer used in the new media UI, there's a plugin that re-enables the old UI.
We should also replace basename() with wp_basename(), as in [22367] and [24585]. See 25236.patch.