function getChangeSetContentType(sMimeTypeHeaders) {
		var sContentType = getHeaderValue(sMimeTypeHeaders, "content-type");

		return sContentType.startsWith("multipart/mixed;") ? sContentType : undefined;
	}
Extracts value of Content-Type header from the specified
<code>sMimeTypeHeaders</code> if it is "multipart/mixed".

@param {string} sMimeTypeHeaders
  Section of MIME part representing HTTP headers
@returns {string|undefined} Content-Type header value e.g.
  "multipart/mixed; boundary=batch_id-0123456789012-345" or <code>undefined</code>