Freedesktop Thumbnail spec - Detecting modifications

I'm following up an issue with thumbnails when renaming image files with KDE/Dolphin, see:

https://bugs.kde.org/show_bug.cgi?id=433127

There were two images, "one.jpg" and "two.jpg". Renaming "two.jpg" to "twox.jpg" worked. But when renaming "one.jpg" to "two.jpg", "one.jpg" picked up the thumbnail of "two.jpg"

The "Detecting Modifications" section of the thumbnail spec should have covered this, except that the files had been copied into a new folder and had the same modification times (to within the second)

Storage and systems are getting faster and it seems quite possible that many files copied, or batch edited to add metadata, would end up with the same modification time.

I think it's worth recommending comparing the file size as well as the modification time