Base StructureDefinition for Attachment Type: For referring to data content defined in other formats.

Attachment
contentType
Mime type of the content, with charset etc.
data
Data inline, base64ed
url
Uri where the data can be found
size
Number of bytes of content (if url provided)
hash
Hash of the data (sha-1, base64ed)
title
Label to display in place of the data
creation
Date attachment was first created