feat: use the original format of images

Author: Asuka109

packages/core/src/image.ts

@@ -113,7 +113,12 @@ export class Image {
       | Jp2Options
       | TiffOptions,
   ) {
-    return this.sharp.toFormat(format, options);
+    this.sharp.toFormat(format, options);
+    return this;
+  }
+
+  toBuffer() {
+    return this.sharp.toBuffer();
   }
 
   clone() {

packages/core/src/loader.ts

@@ -26,7 +26,7 @@ async function process(this: Rspack.LoaderContext, content: Buffer) {
   logger.debug(`Creating thumbnail: ${thumbnail.width}x${thumbnail.height}`);
   image.resize(thumbnail);
 
-  const buf = await image.format('jpeg').toBuffer();
+  const buf = await image.toBuffer();
   thumbnail.url = `data:image/jpeg;base64,${buf.toString('base64')}`;
   logger.debug(`Created thumbnail: ${thumbnail.url}`);