code: function ( url, content, onLoad, onProgress, onError, crossOrigin, materialOptions ) {
  		var resource = new THREE.LoaderSupport.ResourceDescriptor( url, 'MTL' );
  		resource.setContent( content );
  		this._loadMtl( resource, onLoad, onProgress, onError, crossOrigin, materialOptions );
  	}
docs: Utility method for loading an mtl file according resource description. Provide url or content.
  
  @param {string} url URL to the file
  @param {Object} content The file content as arraybuffer or text
  @param {function} onLoad Callback to be called after successful load
  @param {callback} [onProgress] A function to be called while the loading is in progress. The argument will be the XMLHttpRequest instance, which contains total and Integer bytes.
  @param {callback} [onError] A function to be called if an error occurs during loading. The function receives the error as an argument.
  @param {string} [crossOrigin] CORS value
  @param {Object} [materialOptions] Set material loading options for MTLLoader