2.2.3 [RFC5545] Section 3.1.3 Binary Content

V0011:

The specification states that binary content should be referenced using a URI within a property value, but if this is not feasible, then it should be encoded with base64 encoding and included in the iCalendar entity.

When exporting iCalendar information, Microsoft Exchange Server references binary content with a cid: type URI.

When importing iCalendar information, Microsoft Exchange ignores the ATTACH ([MS-OXCICAL] section 2.1.3.1.1.20.1) property. All attachments from the MIME structure of the message are imported.