FHIR documentreference Page
You can say that CDA is application/hl7-v3+xml
There is a large number of CDA already stored using a mime type of text/xml and application/hl7-v3+xml is probably wrong for CDA as well, and hasn't been registered with IANA, and hasn't been through an HL7 process to confirm it, and ...
And you've lost formatCode which was in XdsEntry and XdsEntry2 which is more granular than mimeType and also stored for a wide variety of documents.
Category might be the right answer for formatCode, but there's a notion of conformance in formatCode that category doesn't cover.
So, it doesn't work, and we need to think about it.