multipart 图片文件名按真实内容类型给扩展名

- 新增 multipart_image_file_name，JPEG/WebP/GIF/AVIF 不再统一叫 .png
- 服务端仍按 MIME part 判定，文件名只作为未来消费者可依赖的事实保持一致
- 补扩展名映射单测

apps/ai-game-creator-shell/src-tauri/src/game_distribution_publish.rs

@@ -1162,17 +1162,37 @@ fn publish_multipart_form(
             .map_err(|error| format!("构建发布资料 part 失败：{error}"))?,
     );
     if let Some(cover) = media.cover.as_ref() {
-        form = form.part("cover", multipart_image_part(cover, "cover.png")?);
+        form = form.part(
+            "cover",
+            multipart_image_part(
+                cover,
+                &multipart_image_file_name("cover", cover.content_type),
+            )?,
+        );
     }
     for (index, screenshot) in media.screenshots.iter().enumerate() {
         form = form.part(
             "screenshot",
-            multipart_image_part(screenshot, &format!("screenshot-{index}.png"))?,
+            multipart_image_part(
+                screenshot,
+                &multipart_image_file_name(&format!("screenshot-{index}"), screenshot.content_type),
+            )?,
         );
     }
     Ok(form)
 }
 
+/// multipart 文件名按真实内容类型给扩展名：JPEG/WebP 等不再一律叫 `.png`。
+///
+/// 服务端按 MIME part 判定，文件名只是给未来消费者看的事实；这里保持与 `content_type` 一致。
+fn multipart_image_file_name(part_name: &str, content_type: &str) -> String {
+    let extension = match content_type {
+        "image/jpeg" => "jpg",
+        other => other.strip_prefix("image/").unwrap_or("bin"),
+    };
+    format!("{part_name}.{extension}")
+}
+
 fn multipart_image_part(
     image: &DraftImage,
     file_name: &str,
@@ -1852,6 +1872,23 @@ mod tests {
         assert!(image_content_type("cover").is_err());
     }
 
+    #[test]
+    fn multipart_image_file_name_follows_content_type() {
+        assert_eq!(multipart_image_file_name("cover", "image/png"), "cover.png");
+        assert_eq!(
+            multipart_image_file_name("cover", "image/jpeg"),
+            "cover.jpg"
+        );
+        assert_eq!(
+            multipart_image_file_name("screenshot-2", "image/webp"),
+            "screenshot-2.webp"
+        );
+        assert_eq!(
+            multipart_image_file_name("screenshot-0", "application/octet-stream"),
+            "screenshot-0.bin"
+        );
+    }
+
     fn fixture_root(prefix: &str) -> std::path::PathBuf {
         static NEXT_TEST_ID: std::sync::atomic::AtomicU64 = std::sync::atomic::AtomicU64::new(0);
         let root = std::env::temp_dir().join(format!(