FileStorageApi.File.public_url

You're seeing just the callback public_url, go back to FileStorageApi.File module for more information.
Link to this callback

public_url(arg1, arg2, arg3, arg4)

Specs

public_url(String.t(), String.t(), DateTime.t(), DateTime.t()) ::
  {:ok, String.t()} | {:error, String.t()}