Why should that operation be supporting multipart? You are downloading one specific content stream (or a sub-range thereof) from a document - it only has a single piece of content that is transmitted, so there is no need for multipart in the first place.
You can always read the specification to check if something is supported or not. In there it states that HTTP POST for updating a content stream MUST use multipart/form-data in Browser binding. This is in order to support form-based interaction from web sites. There is no single mention of multipart for download.
