Thanks a lot for this marvellous post! This helped me out. Furthermore, I found out another bug (macOS Sierra 10.12 and an iPhone 7 plus with iOS 10). I assume more people have been hit by this bug.

I recently ‘digitised’ 12 DV tapes from 15 years ago, resulting in lots of large movie files with the extension ‘DV’ (essentially mpeg-files).

macOS has no problem playing them, and macOS Photos has no problem importing them, and uploading them effortlessly into iCloud: it even preserves the original capture dates and times.

Nevertheless, the iOS-syncing trouble starts here, because iOS cannot do anything with these DV-files: it only knows black thumbnails, showing the duration of the movie, but that’s it. Playing them is not possible. This is strange, because downloading them from iCloud from within a webbrowser results in mp4-files, letting me believe that these DV-files were already converted in iCloud.

But they weren't: as soon as I started the Image Capture app, it appeared my iPad Pro 10.5 and iPhone 7 plus had the original DV-files on them. This partly stalled iOS syncing, resulting in showing “adding xxxxx photos” forever…

As soon as I turned off iCloud Photo Library on my iPhone and iPad, deleted these DV-files (and some ‘3gp-files’), restarted the devices, and turned on iCloud Photo Library again, everything started syncing again!