An idea for adding attachments, without blowing up the idea of simplicity:

Add option for connecting an DropBox-Account. Add an area (e.g. below the note field) for dropping files. If a file is dropped, add a (shortened) url pointing to the file in the dropbox to the note - this allows accessing the files from any other client. Within Metanota a click on the file can open a local file copy.

In my opinion this can be a lightweight way for enriching metanota with attachments, without impairing the basic idea of simplenote.

What do you think?