On creation, attachments go into a special _attachments attribute of the document. They are encoded in a JSON structure that holds the name, the content_type and the base64 encoded data of an attachment. A document can have any number of attachments.
attachments are by default stored inline in the document itself
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attachments are by default stored inline in the document itself
That's only when you are initially saving the document. Once the database receives the attachment, it creates a stub and removes the inline data.