fix(feishu): 修复内容类型处理，确保返回字符串格式

src/channels/feishu.rs

@@ -360,9 +360,10 @@ impl FeishuChannel {
             .headers()
             .get("content-type")
             .and_then(|v| v.to_str().ok())
-            .unwrap_or("image/jpeg");
+            .unwrap_or("image/jpeg")
+            .to_string();
 
-        let ext = match content_type {
+        let ext = match content_type.as_str() {
             "image/png" => "png",
             "image/gif" => "gif",
             "image/webp" => "webp",