fix: specify BACnet contentType

[fennibay]

Without specification it falls back to application/json, which is wrong
Now it will default to application/octet-stream, which means arbitrary binary data

BACnet media type #357

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[egekorkan]

I was going to remove the default from the contentType but it actually makes sense if a bacnet TD is processed, where a JSON Schema validator can insert it if needed.

[egekorkan]

We discussed it in the TD call and the group is fine. The new initial connection container will help with this kind of strict contentType protocols by avoiding redeclaring it in each form.