feat(storage): Added cid property to storage files

[andris9]

When creating storage files, you can use cid property

{
  "filename": "image123.png",
  "contentType": "image/png",
  "encoding": "base64",
  "cid": "<unique-value@example>",
  "content":...
  }

This is then included in the generated attachment object and can be referenced from the HTML

<img src="cid:unique-value@example">