fix image error responses

backend/src/main/java/com/petshop/backend/exception/GlobalExceptionHandler.java

@@ -5,6 +5,7 @@ import jakarta.servlet.http.HttpServletRequest;
 import org.springframework.dao.DataIntegrityViolationException;
 import org.springframework.data.core.PropertyReferenceException;
 import org.springframework.http.HttpStatus;
+import org.springframework.http.MediaType;
 import org.springframework.http.ResponseEntity;
 import org.springframework.validation.FieldError;
 import org.springframework.web.HttpRequestMethodNotSupportedException;
@@ -118,7 +119,9 @@ public class GlobalExceptionHandler {
                 request.getRequestURI(),
                 LocalDateTime.now()
         );
-        return ResponseEntity.status(status).body(error);
+        return ResponseEntity.status(status)
+                .contentType(MediaType.APPLICATION_JSON)
+                .body(error);
     }
 
     private String buildDetails(Exception ex) {