That’s kind of stretching the idea of “more structure” to the breaking point, I think. The key is just a string. There is no entry for directories.

> the API implies that common prefixes indicate related objects.

That’s something users do. The API doesn’t imply anything is related.

And prefixes can be anything, not just directories. If you have /some/dir/file.jpg, then you can query using /some/dir/ as a prefix (like a directory!) or you can query using /so as a prefix, or /some/dir/fil as a prefix. It’s just a string. It only looks like a directory when you, the user, decide to interpret the / in the file key as a directory separator. You could just as easily use any other character.