"Other Items" in iCloud Photos

On MacOS 11 Big Sur or Catalina or Mojave you can use the "Showing Only" filter (Show specific types of items in Photos on Mac - Apple Support) to show only photos.

• Set the Filter to "Photos".
• Now you can select all photos at once with ⌘A and create a new album named "Photos Only" from the selection ⌘N.
• Now you can create a smart album with two rules and combine them with "Match All":
  • Album > is not > Photos only
  • Photo > is not > Video

All items in this smart album are likely candidates for the problematic items. Look there for files with unusual filename extension.

Alternately, try to weed out "items" with a smart album, where you exclude the files based on the filename extensions for image file types that are working well and see what ist left, as described here: How to Weed out Legacy Media in Photos for Mac - Apple Community

"Other items" can be anything, uploaded to iCloud Photos from a device with an older system version, that is neither recognised by Photos an image file nor a video. When I first started to use iCloud Photos, many of the files counted as "Other" have been audio files. The audio files in my library have been imported into Aperture as sound tracks for slideshows. After I migrated my main Aperture Library to Photos for Mac, the audio files appeared as "Other".