MM-9747: Small fixes for attachments import (#9225)

* MM-9747: Small fixes for attachments import

* Adding unit test RootId check in replies

api4/file.go

@@ -285,6 +285,7 @@ func getFilePreview(c *Context, w http.ResponseWriter, r *http.Request) {
 	if err != nil {
 		c.Err = err
 		c.Err.StatusCode = http.StatusNotFound
+		return
 	}
 	defer fileReader.Close()