add(partName, contentType) {
        const node = {
            name: "Override",
            attributes: {
                PartName: partName,
                ContentType: contentType
            }
        };

        this._node.children.push(node);
        return node;
    }
Add a new content type.
@param {string} partName - The part name.
@param {string} contentType - The content type.
@returns {{}} The new content type.