fix: skip thumbnail generation for GIF images (#6597)

#### What type of PR is this?
/kind bug
/area core
/milestone 2.20.x

#### What this PR does / why we need it:
修复 GIF 缩略图生成只会保留第一帧的问题

#### Which issue(s) this PR fixes:
Fixes #6596

#### Does this PR introduce a user-facing change?
```release-note
修复 GIF 缩略图生成只会保留第一帧的问题
```

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

@@ -13,12 +13,14 @@
 import java.nio.file.Path;
 import java.util.Iterator;
 import java.util.Optional;
+import java.util.Set;
 import javax.imageio.ImageIO;
 import javax.imageio.ImageReader;
 import javax.imageio.stream.ImageInputStream;
 import lombok.AllArgsConstructor;
 import lombok.extern.slf4j.Slf4j;
 import org.imgscalr.Scalr;
+import org.springframework.lang.NonNull;
 
 @Slf4j
 @AllArgsConstructor
@@ -29,6 +31,8 @@ public class ThumbnailGenerator {
      */
     static final int MAX_FILE_SIZE = 30 * 1024 * 1024;
 
+    private static final Set<String> UNSUPPORTED_FORMATS = Set.of("gif", "svg", "webp");
+
     private final ImageDownloader imageDownloader = new ImageDownloader();
     private final ThumbnailSize size;
     private final Path storePath;
@@ -70,13 +74,16 @@ private void generateThumbnail(Path tempImagePath) throws IOException {
         if (file.length() > MAX_FILE_SIZE) {
             throw new IOException("File size exceeds the limit: " + MAX_FILE_SIZE);
         }
-        var formatNameOpt = getFormatName(file);
+        String formatName = getFormatName(file)
+            .orElseThrow(() -> new UnsupportedOperationException("Unknown format"));
+        if (isUnsupportedFormat(formatName)) {
+            throw new UnsupportedOperationException("Unsupported image format for: " + formatName);
+        }
+
         var img = ImageIO.read(file);
         if (img == null) {
-            throw new UnsupportedOperationException(
-                "Unsupported image format for: " + formatNameOpt.orElse("unknown"));
+            throw new UnsupportedOperationException("Cannot read image file: " + file);
         }
-        var formatName = formatNameOpt.orElse("jpg");
         var thumbnailFile = getThumbnailFile(formatName);
         if (img.getWidth() <= size.getWidth()) {
             Files.copy(tempImagePath, thumbnailFile.toPath(), REPLACE_EXISTING);
@@ -87,6 +94,10 @@ private void generateThumbnail(Path tempImagePath) throws IOException {
         ImageIO.write(thumbnail, formatName, thumbnailFile);
     }
 
+    private static boolean isUnsupportedFormat(@NonNull String formatName) {
+        return UNSUPPORTED_FORMATS.contains(formatName.toLowerCase());
+    }
+
     private File getThumbnailFile(String formatName) {
         return Optional.of(storePath)
             .map(path -> {