The attachment.data data type component uses data type binary64.  When mapping from HL7 v2 MDM messages to FHIR, it is permissible to use in OBX-5.4 "A" (ASCI) or "HEX" as well.  We seek clarification whether in FHIR we should either:

provide guidance that "A" or "HEX" data should always be converted to binary64

use an extension to support .data[x] of string or HEX (noting that HEX is not a recognized data type in FHIR, so this would require more updates to support that extension format).

We are open to either and will follow FHIR-I guidance.