XHRLoader: Check if overrideMimeType is available before using it. Fixes #9376.

src/loaders/XHRLoader.js

@@ -38,7 +38,6 @@ Object.assign( XHRLoader.prototype, {
 		}
 
 		var request = new XMLHttpRequest();
-		request.overrideMimeType( 'text/plain' );
 		request.open( 'GET', url, true );
 
 		request.addEventListener( 'load', function ( event ) {
@@ -95,6 +94,8 @@ Object.assign( XHRLoader.prototype, {
 		if ( this.responseType !== undefined ) request.responseType = this.responseType;
 		if ( this.withCredentials !== undefined ) request.withCredentials = this.withCredentials;
 
+		if ( request.overrideMimeType ) request.overrideMimeType( 'text/plain' );
+
 		request.send( null );
 
 		scope.manager.itemStart( url );