Give name to the #attachment's anonymous module

One RubyTapas motivated me that whenever I'm creating an anonymous module like this, to always try to make the ancestor chain prettier.

Before:

```rb
class User < ActiveRecord::Base
  attachment :avatar
end

User.ancestors #=> [User(...), #<Module:0x007fd08af414d0>, ...]
```

After:

```rb
class User < ActiveRecord::Base
  attachment :avatar
end

User.ancestors #=> [User(...), Refile::Attachment(document), ...]
```