test: Change content-type to text/plain

Author: reddevilmidzy

tests/file_manager/handlers.rs

@@ -154,7 +154,7 @@ async fn get_file_success() {
         .unwrap()
         .to_str()
         .unwrap();
-    assert_eq!(content_type, "text/html; charset=UTF-8");
+    assert_eq!(content_type, "text/plain; charset=UTF-8");
 
     let file_content = response.text().await.unwrap();
     assert_eq!(