What are "Items" in OS X Photos? In my Photos library, there are some mysterious "items" that are not Photos/Videos. What are they?

My hunch is that they may be corrupted/unsupported video files, but I couldn't find a way to locate them and/or remove them :(
Turns out those are some .mp3 files- probably got carried-over from my Aperture library ¯_(ツ)_/¯