Is there a convention which MIME_type to use for 3dm files? The official list maintained by IANA doesn’t include it. Other sources like this one suggest to use
x-world/x-3dmf, but my suspicion is that this was meant for a different file format. It would be good to get a general agreement on this, because an increasing number of services will serve 3dm files via APIs and HTTP, ie the Content-Type header should be set to sth meaningful.
model/3dm would be a good choice, as it would align with existing types
model/iges, etc (as defined in the official list by IANA).