The mostRecentAttachment is a separate and redundant instance with whats in the
media gallery item cache.
Alternatively, we could double down on the one-instance-per-value paradigm by
creating a way to manually add existing instances to the cache, but the value
semantics proposed here seemed more natural.
I wish we could have just made RailViewItem: Equatable, but alas "protocol can
only be used as a generic constraint because it has Self or associated type
requirements"
