iCloud Drive support means using Apple’s APIs for detecting files and downloading them as needed. Thumbnails may be available without a full download, but I don’t know yet. However, if a file has been edited on machine A over iCloud Drive, and then you view it on machine B, the thumbnail will be still be the original image’s thumbnail until the app can download the original and apply the edits to make a local thumbnail. The thumbnail *could* be stored in the XMP, but XMP is a terribly inefficient format for large binary data, so I don’t put thumbnails there. External / network devices don’t go through the same system as iCloud Drive, so changes for iCloud Drive would not have any effect on the NAS case.