It can be a bit of a mystery, but they've used a simple pattern.
by John Norris
You can find the exact file that is attached to your model locally by looking in the storage directory at the root of the app.
What isn’t clear is where you go from storage.
Rails groups all the files into directories based on the attachment’s key, where the first two chars of the key are the first directory, and the second two chars are the subdirectory.
E.g. if you have a project with a document attached
project.document.blob.key # -> 2dkawl12g1otdcnqo16bqzk5m67a
the actual file will be
storage/2d/ka/2dkawl12g1otdcnqo16bqzk5m67a
It was very helpful just now when I wanted to test purging the file and ensure it actually deleted afterwards.