[release-2.19] refactor: preserve original image if smaller than requ…

…ested thumbnail size (#6583)

This is an automated cherry-pick of #6582

/assign ruibaby

```release-note
当生成缩略图时如果原图尺寸小于请求尺寸则返回原图以保持其质量
```

application/src/main/java/run/halo/app/core/attachment/ThumbnailGenerator.java

@@ -1,5 +1,7 @@
 package run.halo.app.core.attachment;
 
+import static java.nio.file.StandardCopyOption.REPLACE_EXISTING;
+
 import java.io.File;
 import java.io.FileOutputStream;
 import java.io.IOException;
@@ -74,10 +76,14 @@ private void generateThumbnail(Path tempImagePath) throws IOException {
             throw new UnsupportedOperationException(
                 "Unsupported image format for: " + formatNameOpt.orElse("unknown"));
         }
-        var thumbnail = Scalr.resize(img, Scalr.Method.AUTOMATIC, Scalr.Mode.FIT_TO_WIDTH,
-            size.getWidth());
         var formatName = formatNameOpt.orElse("jpg");
         var thumbnailFile = getThumbnailFile(formatName);
+        if (img.getWidth() <= size.getWidth()) {
+            Files.copy(tempImagePath, thumbnailFile.toPath(), REPLACE_EXISTING);
+            return;
+        }
+        var thumbnail = Scalr.resize(img, Scalr.Method.AUTOMATIC, Scalr.Mode.FIT_TO_WIDTH,
+            size.getWidth());
         ImageIO.write(thumbnail, formatName, thumbnailFile);
     }