add y to with-aspect naming schema

please review @icewind1991 @MorrisJobke 

@icewind1991 I am not sure how to properly cleanup the existing cached previews that already have this {x}-with-aspect naming schema. Is there some good way to get all files where the filename matches a regex?

partial fix for https://github.com/owncloud/core/issues/10570
Please add PHPDoc.