Question: In a Multimedia Broadcast/Multicast Service (MBMS) metadata envelope, what is the primary purpose of the `contentType` attribute, and when is it mandatory?
Option 1: To specify the encoding scheme; always mandatory for both embedded and referencing envelopes.
Option 2: To indicate the MIME type of the metadata fragment; mandatory only for referencing metadata envelopes.
Option 3: To define the versioning system; optional for all types of metadata envelopes.
Option 4: To specify the MIME type of the metadata fragment; mandatory for embedding metadata envelopes but optional for referencing ones.
Explanation: The `contentType` attribute specifies the MIME type of the metadata fragment, crucial for handling it correctly. It's mandatory for embedded metadata envelopes to identify the fragment's type clearly but optional for referencing envelopes, where the type can be determined upon fragment retrieval.