// First, we compute the ETag of the multipart object.
	// The ETag of a multi-part object is always:
	//   ETag := MD5(ETag_p1, ETag_p2, ...)+"-N"   (N being the number of parts)
	//
	// This is independent of encryption. An encrypted multipart
	// object also has an ETag that is the MD5 of its part ETags.
	// The fact the in case of encryption the ETag of a part is
	// not the MD5 of the part content does not change that.