The MULTIMEDIA data type displays a list of types, with an option to uncheck some to restrict the allowed types. However the displayed list is only partial and also does not allow adding new types.
For example application/json is not in the list and cannot be added. This list should be either the complete IANA media type list or should be kept unconstrained
The media type terminology should be considered as an external terminology since it’s not maintained by openEHR, though recently openEHR did update it’s local “copy” of the terminology, though I personally think it’s a bad idea because we need to keep up with the changes done by IANA. And IMHO tooling shouldn’t depend on the openEHR’s copy of the IANA terminology.