2.2.3 [RFC5545] Section 3.1.3, Binary Content
The specification states that binary content should be referenced using a Uniform Resource Identifier (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.
Microsoft Office Outlook 2007, Microsoft Outlook 2010, Microsoft Outlook 2013, Microsoft Outlook 2016, Microsoft Outlook 2019
When exporting iCalendar information for use in a MIME message, Outlook references binary content with a cid: type URI. When exporting iCalendar information to a file, Outlook encodes binary content with base64 encoding.
When importing iCalendar information, Outlook imports all binary content that is encoded with base64 encoding. All non-cid: type URIs are imported as .url file type attachments. iMIP data is also imported with all attachments from the entire MIME structure, regardless of whether they are referenced by cid: type URIs. All cid: type URIs are ignored.