GLTFExporter: Cleanup. (#23385)

examples/jsm/exporters/GLTFExporter.js

@@ -1050,7 +1050,7 @@ class GLTFWriter {
 		if ( ! cache.images.has( image ) ) cache.images.set( image, {} );
 
 		const cachedImages = cache.images.get( image );
-		const mimeType = format === RGBAFormat ? 'image/png' : 'image/jpeg';
+		const mimeType = 'image/png';
 		const key = mimeType + ':flipY/' + flipY.toString();
 
 		if ( cachedImages[ key ] !== undefined ) return cachedImages[ key ];