Is the instance variable `@upload_path` used anywhere? Right now we just assign to is whenever `upload_path` is called, but it doesn't seem to me like we ever read from it. This should probably be:

``` Ruby
def upload_path
  @upload_path ||= if …
  end
end
```

However, I think this method could be a lot more concise if we consider the file name part of the path:

``` Ruby
def upload_path(filename)
  [options.fetch(:upload_dir, ""), filename].join("/")
end
```

What do you think?